I'd say you need to decide, first of all, whether you want an SLR or RF. Both have their pros and cons. SLRs tend to be bigger, heavier, noisier and so on but they are less hassle with things like framing accuracy and for extreme close-up they're a far better tool. RFs are a bit more of an acquired taste but they do have advantages for some uses.
From the 60s to 80s there are numerous manual/AE SLRs, stick to a decent brand, such as Pentax, Minolta etc and you can probably pick something up for not too much outlay and have access to a good selection of lenses.
If you really want an RF, again there are plenty of choices. Consider whether a fixed-lens will do, if not then decide if you need framelines for different lenses or you'll be happy to use an external finder and check what range of lenses is available and at what cost. Will you want a built-in meter or automation or manual-only? Many cameras from the earlier eras will not have working meters by now. Searching the threads on here will give you plenty of suggestions for RFs, just remember that asking a dozen people will get you a dozen different opinions!
Any camera of that age will be a gamble on service-history or may need attention. Most will need, at least, replacement light-seals (if there are any foam ones especially). Cameras that are used tend to fare better, don't go for the pristine model that hasn't seen much use unless you know it's because it had a loving owner and had regular but low use.
