as an owner of 3 reel-to-reel machines and a fourtrack cassette machine, all acquired used, i can say the biggest concern is finding one that actually works.
i was given one reel-to-reel that works almost flawlessly except one of the 8 tracks doesn't work. another machine i bought at an estate sale for cheap, it seemed to work when i tested it there, but got it home to find out it has some capstan or belt problem. another machine i bought from an old studio closing sale and it SEEMS to be ok after minimal testing.
my 4-track cassette portastudio was bought on ebay, claiming to work, but turns it out needed a new pinch roller, which i managed to order from Tascam an install myself.
so, so far, so good... but i don't know if i'd ever buy another tape machine used that didn't say "newly serviced" or "just gone over by tech" or "100% working, new belts" etc... unless you're a handy DIY guy (which i'm not really) you'll often be into a repair project.
if you're just looking for a processing type of tool instead of a full-featured recorder to mix down to, then you should probably go for the "worst" model you can find. high quality reel-to-reel machines can be really clean and hi-fi sounding... so if you want something for effect i'd go for old and low-end... of course, those will also tend to have the most problems...
good luck, and keep us posted!