Some very good suggestions here to the OP. What is your primary photo interest? It may be pertinent to the suggestions. If looking for macrophotography, a TLR is pretty much impractical, thought it is possible. For landscapes and still life, a TLR is as good as almost anything provided a fixed lense camera (except the Maymiya TLR) is acceptable. A modular SLR allows for the most flexibility if varied interests. There are also rangefinder cameras in both the fixed lens and multi-lens formats. A lot depends on what features you want, how it will be used and how it fels to you.
For instance, if pack packing and no tripod, a heavy camera will soon make yolu think your choice. If in a studio setting, weight is of little concern except in making sure a tripod can support it. If a beginner, it may be better to restrict youself to a fixed lens body and learn to us the lens and things like using your feet to change perspective, DoF, etc. I've seen a number of potentially decent shooters not learning the basics and the results are what could have been an excellent photo turned out to be trash.
To clarify, generally a cropped 6x6 to make it rectanglular, will yield about the same image coverage as a 645. 6x6 is an interesting format and while some find it difficult to master, once done, it is no more difficult to work with than any of the rectanguar formats.
Older vs newer cameras, are less an issue. It depends on how much you are going to shoot and the camera condition. I've seen old clunkers in better operating condition than the new stuff. However, to be on the safe side, old workhorses should at least have their shutters checked and folders, their bellows. Cost for the cameras vary but very good ones with legendary lenses can be had very reasonable and depending on the shutter, a tech check can be very reasonable. In the past there were many types of cameras not available later such as folders with lenses that were legendary. Amongst these the Zeiss and Voightlander names are legendary. In the TLR lineup of course there are the Rolleiflex and Rolleicords.
I found no one camera fits all for me so ended up with a few of them. My primary system is a Bronica ETRS bought back when they were in production along with a number of accessories. The basic camera has the WLF what like a TLR gives a lower shooting position than the SLR generally. Today, with the standard 75mm len, this camera is less than most 35mm SLRs in the better amatuer/pro category. The lenses are excellent regardless of which series, main difference being the coating and f/stops. Accessories include prism, metered prisim, motor drives, and a speed grip. With the metered prism and speed grip, it handles and feels like a 35mm slr and is not that much bigger than some of the pro level 35mms. Add a 35mm back and it makes an excellent 35mm SLR that includes the ability to use different film loads (depending on how many backs you have). The standard 645 back adds no weight. The system includes a very wide range of lenses, including some zooms and a tilt and shift, macro extenders, focusing screens, and many more things. While when in production, the system was not inexpensicve, today it is. The last think I bought was the 105mm / macro lens for $100 and a prism finder (non-metered) for $90. In my system the only thing I'd like to add would be the zoom lenses and the 500mm but as I have the doubler for my 250mm, not that important.
But, while it gives me both a 35mm and MF system with commonality, I find it a bit bulky to carry every days and use either my Leica CL or a 35mm Zeiss Ikonta as a daily carrier. I'd leave none in my car so in the glove box, I have a 35mm Canon rangerfinder 35AF. On days when I think I might find some interesting scenery or something but don't want to schlep the Bronica or on days when I\d carry the Leica or Zeiss but not going hither and thither so I can relax, I'll sling the Rolleiflex TLR.