i'm thinking about buyinga used car but a lot of used cars for sale have high miles. the ones with relatively low miles either have engine or some major part replaced or are listed as rebuilt/restored title. so, before i buy a one, i need clarification on some questions regarding used cars:
1. is it wise to buy cars which have engine/motor/transmission replaced? why/why not?
2. what kind of cars fall under rebuilt/restored category? are cars with replaced engine/tranny/motor considered rebuilt/restored? 
3. what are goods/bads of buying a rebuilt/restored car? in general, is it wise to buy a rebuilt/restored car?

thanks folks.