I've gotten mine through personal sales only, and I've had half-decent (but not perfect) luck.
For what it's worth, I've planned on several jobs as soon as I buy mine- all new brakes, all new struts, replace the Intake Manifold Gasket, flush all fluids, etc. - this whole thing takes a couple of days but then you proactively eliminate 90% of the maintenance-related problems down the road. I'd recommend the same for you, especially if you have a garage.
Issues like "very little heat" can usually be cured pretty simply, and so can a car full of dog hair (unless all of the fabric was torn). So it might be worth having potential purchases analyzed by a mechanic, then come back to the board and advise us of what he's found so we can tell you know whether the problems are big or small. Also, there are quite a few older cars with "siezed engines" which just need an intake manifold replaced - they can be purchased at a huge discount and repaired in a day (if you have a garage).
-BC
