The main places to look for rust are around the rear window, trunk, and lower quarter panels.
Then lower front fenders, lower doors, and front foot wells.
Under the hood, look around the hood hinge area on the inner fenders.
Prices I think are fairly high. Pretty poor shape cars that need almost everything are pretty expensive, often around $10,000, but you usually know what your starting with will need work.
With repainted or "restored" cars, you need to carefully inspect the quality of the work. A car might have a cheap paint job over a poor body, and to get it right you might have to un-do, and start over. The thing is all in the cost. If paying good money like $30,000+ the car better be in very good condition or have expensive upgrades.
Most of my cars don't look the best, but have expensive drivetrains, but without proof, you can't tell from looking at the outside what all is inside an engine, transmission, or rear axle. Like automatic transmissions, from the outside looks it is hard to tell if it is a stock $300 transmission or a race built $5000 transmission (OK, the race trans likely will have clues, like shield/blanket, manual valve body, maybe a trans brake, plugged speedometer hole, or re-located vent).
Engines are even harder to know the cost of what was put into them, but for a buyer, the performance is what will matter.
I like modifying my cars, so I look for poor or non-running cars with good bodies which can save me money over a decent running car that might need body work.
