Hrm... guess no one heard of a tripod (or monopod for that matter).
Autofocus is obviously going to be more expensive than the manual focus options, and there are plenty of manual focus bodies out there for very inexpensive price.
I ended up getting a Mamiya RB67 + prism + 90mm f/3.8C and so forth... it ain't light, and I have shot it for several hours during an event handheld before. I plan to use it again this year, except I'm preped with a carbon fiber monopod and a good ball head that can handle the weight... and guess what, with the ball locked, and due to the heavy weight of the camera against the monpod, and propped to my face due to the eye-level viewing, I've done 1/8th of a second without a problem.
But for inexpensive autofocus, I'd have to lean towards either the Pentax or Mamiya... the Rolleiflex 6008 and similar bodies while nice are most certainly more expensive than the previous two brands. You could shoot with any of them, its a matter of personal preference and knowing what your limitations are.
It also helps to know a few things ahead of time.
What format.. do you want 645, square (6x6) , or do you wish to go bigger like 6x7.
Is weight much a concern do you see yourself shooting outside a lot or without a support of some sort.
Is size of camera a concern, how you plan on traveling with it.
Are you planning on using available light or will you at any point of time consider strobes or flashes and such.
Have you ever had a chance to find a shop that would have these cameras to pick up and hold and feel.
I chose the RB67 for two primary reasons, a BIG negative, and damn CHEAP now days for the camera, lens and it's accessories. The RZ67 is a newer lighter one, tiny bit more expensive, but is the next step up if you wish to slide more towards keeping a big negative, but being somewhat bias towards weight limitations.
A second recomendation would be something like the Bronica SQ-A or SQ-Ai (main difference is an Ai has bulb mode, timed shutter upto 16 seconds as opposed to 8, and has an Flash TTL Metering against the film plane, as well as being compatible with a motor winder, but the SQ-A is quite a bit cheaper, you can get a good outfit for 400$ or less at keh.com) , the nice thing about the Bronica SQ is that they are 6x6 square format, but handles like most 645 cameras, and if you use a prism with them (which most outfits are sold with prisms) a speed grip is a very nice accessory to have, otherwise if you use the WLF, not having a grip works ok. This would been my second choice if I didn't go with the Mamiya RB67 mainly because its inexpensive for what you get and is still larger than 6x4.5, and you still got the square composition in mind that you've already experienced with a TLR.
Oh in case I did not mention earlier, my Mamiya RB67 + 90mm f/3.8C lens + 120 Back + prism, is about 12 pounds in weight, so keep that in mind.