wait, ur question is 'HOW to choose a nice 2nd hand car' right?
the other questions you can seek later....via the 'SEARCH' button.
ok,
to check for serious accidents, the easiest thing is to bring a strong enough magnet and start sticking it onto every possible part of the car. this is to ensure that the car is completely metal...no putty due to major accidents la. coz since the putty used to smoothen out the unevenness on the body after knocking or welding or whatever, the magnet won't stick onto the body.
another thing is to open the bonnet and check the left/right fenders for creases, even the radiator part, check for knocking or rust UNDER/SIDES and all. just look. take your time.
lift up the carpets to check for stains. and also, crawl under the car to look for tar. this is normally the work of the mudguard lou la.
other ways you can refer to motor trader for more tips. :)