If I understand correctly, the OP has some issues with the camera body. If the lenses are fine, why recommend a new system? Having the body fixed and CLA'd is probably the safest option, in many ways.
About the 553ELX:
I had one. It is a lovely, yet somewhat bizarre camera. And the youngest and probably most reliable of the motor-driven Hasselblads. I bought it when they were just several hundred EUR (body only). Compared to its predecessors it is using standard 1.5V batteries instead of Hasselblads proprietary accus. Yes, there are workarounds for the older models. 553 ELX is just "as it should have been from the beginning".
It is quite small for a MF SLR with motor, then again it is quite hefty. You can use it hand-held and then again it will be getting on your nerves quickly. The novelty of schlepping it around for street photography quickly wore off. I also had the metered prism and it added another hefty weight to the already heavy camera. I would say that a 553ELX with prisma and a 150mm lens is a perfect portrait / model kit for studio use, if that is what you want to do.
All lenses have their own shutter that needs to be looked after. Once you have your Hasselblad system up and running, it is very rewarding. But it is an emotional choice. A motor-less body is more expensive (but not much more, now that 553 prices picked up), but easier to carry, if that matters.
Due to Hasselblad's modular architecture (even the winding knob can be exchanged, there are several different versions of the focussing screen and so on), you might be getting sucked into never-ending loop of upgrading, exchanging, fixing components. I have been through it.
Keep in mind that most Hasselblads are
naked, fully manual cameras, unlike your quite modern Pentax 645. Is that what you are looking for?
I made the mistake of buying cameras based on other people's recommendations instead of going to a shop and playing with lenses and bodies. Of course, that requires some effort (travel, or mail order with the option to return the camera).
One "mistake" was that I did not realize that the original focus screens are quite dark, and if you combine them with, say, a 4/150mm lens (great for portraits), that makes a sunny-day-only camera which is very difficult to focus. Even the 2.8/80 standard focal lens might surprise you when mounted on a HB with an older focussing screen, hence the Acute Matte (or any modern aftermarket) is key. Otherwise you will be disappointed.
I had 3 lenses (50/80/150), initially starting with the old silver ones, then the black ones with the scalloped focus rings and then the ones with the fine knurled / rubber covered focus rings. I found the older lenses uncomfortable to handle; the scalloped focus rings have a long throw and somehow cut into my finger. I simply did not gel with them. So, buyer beware and try before you buy.
About some of the recommendations here:
They all come from their owners' perspectives. A Mamiya Press is very big, very niche and very old (the final model was discontinued in the 1970s), and a rangefinder, not what you asked for. Mamiya 6 is a rangefinder, much younger, and also probably exceeding your budget. TLRs usually have fixed lenses and viewfinders (yes, there are few exceptions and workarounds). Your 645 has a prism; unless you are interested in the quite different WLF experience, its not what you want. Of all TLRs, the Roleiflex GX has a built-in TTL exposure meter, and is likely to exceed your budget. Bronicas are quite niche, with some known quirks, and approaching Hasselblad $$$ by now. Your 645 is probably one of the most modern MF SLRs (it was launched around the same time when the 553ELX was discontinued), and most of the suggested options would be quite a big step back in time and functionality.
To wrap it up: if you love your Pentax, just solve the body issue and keep on shooting. Or if you are hungry for something entirely new, think about what you want from it.
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P.S.:
If you want to buy a Hasselblad, I would be happy to "review" the offers you are looking at - often sellers create a "Franken-blad" our of components from different eras, e.g. old waist-level finders on newer bodies etc. I can help to you decypher what you are actually looking at.
An example of a kit that I would buy (I have no relationship with the seller btw):
Inkoop / Verkoop. - Vintage camera store in Delft - Good working Hasselblad 553ELX kit with CF Planar 80mm f/2.8 T* and a A12 back, fully in black finish. The camera kit is in excellent shape with just some light wear. The camera body is perfectly working and comes with a Acute matte focusing...
fotohandeldelfshaven.nl
The lens and WLF are from the same era as the body, and the seller mentiones the build year of each component (all are coded and you can easily find out when a body, lens or film back was made). The seller explicitly mentioned the desirable (and period) bright Acute-matte focussing screen (often sold separately). From my own experience, 553 bodies sell for 400-600 EUR (depending on condition and history), 80mm lenses usually go for 600-900 EUR, and a A12 film back is 150-250 EUR. Add some margin for the dealer (for warranty and all) and you end up where we are here. Can you save some $$$? Sure, by buying from a private seller. It that like for like? Well, unless they offer some sort of return policy, otherwise no.
I would want a chrome example, not a black one, but that is just my personal preference.
Let's look at another offer:
Hasselblad 553ELX (Chrome, 10067) Manual focus 6x6 V-mount medium format SLR camera introduced by Hasselblad in 1989. This is an improved version of the ELX with better internal reflection coating (palpas), 5 AA batteries as a power source, and an acute matte focusing screen. Metal body...
kamerastore.com
Period-correct WLF with plastic button, but old-style darker focussing screen. Upgrading to Acute-Matte is 200-350 EUR. Add a A12 magazine (150-250 EUR) and a lens of your choice (modern 80mm most expensive, chrome 150mm is cheapest).
That dealer also has an older "EL" (minus "M" which means you cannot easily exchange the focussing screen). Probably the least desired (not "desirable") Hasselblad body, reflected in its price.
This one looks good (seller is in NL though):
De Hasselblad 553 ELX met cassette is een topklasse mediumformaat filmcamera. Met zijn betrouwbare elektronische sluitertijden (tot 1/500 seconde) en geavanceerde belichtingsfuncties, kunt u uw fotografie naar een hoger niveau tillen. Inbegrepen is een handige opbergcassette voor gemakkelijke...
www.profotica.nl
Here is another good example, unfortunately the auction is already over, but it gives you an idea "what good looks like":
with acute-matte D screen and magazine A12 30074 no.30ET4446, connecting cord LK150 and release cord FK300, all new in original maker's boxes, brochure
www.leitz-auction.com
I also looked at the offers on KEH's website. It is difficult for me to judge them correctly, as the photos do not seem to resemble the actual camera on offer. But by now you already know that to the "naked" body (excluding WLF and focussing screen and film back) you have to add a lot of components.
Bottom line:
The 553ELX is a very special camera. Niche, yes. At its time, it was one of the expensive offerings in Hasselblad's product range. Looking at a 1996 price list, it was 40% more than the 503 body! It is a charming camera with a somewhat limited use case.
I would buy a complete kit from a reputable seller (with a decent return policy), and go for a late model (e.g. 553ELX) which has all the correct accoutrements.