Simon, one thing that noone's mentioned is that many of the SLRs will allow you to use their kit on DSLRs. If you're considering one of those, make sure your lineups match. Also be aware that 1980ish most of the brands changed their mount, in most cases the older lenses won't match anything anymore.
Off the top of my head:
Canon FD - no forwards compatability.
Canon EF - works fine with modern Canons.
Nikon - never really changed it? And therefore even really old Nikkors fit to high level Nikons?
Minolta MD - converts easily to 4/3 mount
Minolta AF - works perfectly on KM/Sony DSLRs
Olympus OM - apparantly converts easily to Canon AF
Pentax - never changed their mount. All Pentax lenses fit all pentax cams (going forwards at least)
m42 - any lens with an M42 mount will mount very easily to any modern Camera.
Quick provisos, some DSLRs will allow you to use (e.g. meter with) the old, manual lenses, some won't. Generally the further you get up the range, the better the compatability, e.g. Nikon D40 can't make use of anything, D80 makes limited use, D200 and up you get full functions. But there are exceptions - Sony's A100 lets you use everything, so does their A700.
When I say use everything... Don't expect auto aperture control or auto-focus on lenses that never had it (again, exceptions, manual focus Minoltas can be used with a very rare converter that gave them auto-focus and auto-aperture) You'll have to manually focus and use something called 'stop-down metering' - you manually close the aperture blades and the camera makes the exposure based on the light actually coming through, rather than holding the blades open (so you can actually see...) and then calculating how much light will come through should you close the blades to whatever size.
Did you ever say that you wanted a really old school 50s camera? Film is sooooooooo cheap now, I got a Dynax 7, one of the best film cameras ever made with AF that features the kind of double cross sensor that's only just made its way back into the latest round of 40D/A700, eye-start and auto-rotating LCDs still missing from most contemporary models and muti-segment exposure preview that you can't even get today for a mere £120. People are giving these things away. Why don't you get something that was pretty modern in say 2000, but got rendered obselete overnight with digital? Use it manually if you want to, and if you get something decent you'll find the build quality is still there, my 7 is much tougher and better put together than my 7D.
Finally, quick reccomendation, if you're happy buying into the Alpha mount, an 800si + 24-50/4 will get you a fully zoomable flash head(!), autofocus that beats the pants off a 400D/D40 etc, wireless flash capability, 5fps shooting (although that gets expensive with film) and a very solid build quality (back in the days of film it was a step up from a £700 Alpha 700) for around £80. The lens is solid.
I feel you on the film thing - I started with digital and have just begun to mess around with film, it's got a certain look to it that can be really tough to achieve in PP, and it's got tons of exposure latitude which comes in really handy for certain situations. I got myself an old Nikon coolscan II for £60 which deals fine with B&W and I'm away with ilford FP4 £2 per roll