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.

 In general if you are going to buy a car look for a car that you can afford that has the lowest milage. Always remember a car not only has engine and transmission. So when you buy a car that has high mileage but has repaired engine/transmission, there is always chace that the other parts of the car will break down. 
and answers to your questions;
1. it depends. If its a low milage car go for it, other wise its a bet.
2. Rebuilt restored may be any component of car mechanical or the body. Say the car has front end collision and it gets repaired its been restored.
3. Repairs these days in my city cost $90 per hour and cost per parts. Some cars might run for hundreds and thousands of miles while other could break tomorrow.

Make sure you check the title of used car before you buy. Usually the car title issued has "salvaged vehicle" if the car's been restored after accident.
Have a mechanic handy. Do not trust a friend who has a car to buy yours. While he might have a good advice, a mechanic will lift up the car anc check its internals.

Finally my advise to you is that do not buy a crappy car because you have less money. Save for some more time and get a decent car. You'll save the money later in repairs.

Do not buy salvage or reconstructive title car. Hard to sell back.
If starts giving you problem you may end of paying more than a decent car. Mechanic are well known by butcher.
Regarding tranny or transmission, some car has faulty and after certain mile it has with most cars in that model and year. Other than that it may be a sign of not a regular maintenance or rough driving ? If that is the case it may have some other problems too.
Google , what to look on used car. If you see a shinny engine on 1997-1998 car , it must be degrease and steam washed, too good to be true. I had seen people washing their engine.
Good luck .