I must say of the Canon cameras I got, 350D(New), 30D(New), 40D(2nd Hand), IDMK4(New), I can only speak good of Canon. Probably the most reliable items I had bought so far, a new German car made me walk home quickly as example (The flatbed type).
1-Only problems I had was 1DMK4 had a rare problem(Focussing-Different white lenses). If I try to take a photo slightly againtst the afternoon sun in summer 17:00 with a green tree involved I battled to get it to focus, it was a bit worse in low light as I would expected. Have I had not some experience, I would not have picked this up. The well known place I bought it from replaced it immediately and problem solved.-Zero problems with this one.
2-The 40D was 2nd hand from family member. The only problem today is the shutter button acts intermittantly, however I have a grip on it and that button workd 100%.
So I am try to tell you its difficult to give advice on 2nd hand cameras.......If you can go through all the settings, lenses, extra settings, understand that, and it comes out clean its probably ok.