Any tips on checking if it's straight?http://sheldonbrown.com/frame-spacing.html
would most off the shelf parts fit on an older frame or would I need to source second hand retro parts?
There are quite a few variations for hub width, headset size and bottom bracket widths and threads, but 90s onwards MTB frames were pretty standard.
You can fit new or old parts, the headset will either be 1" or 1 1/8" and both are still readily available in threaded and unthreaded variations. You will need forks to suit the same as well. There are also a couple of bottom bracket shell widths, either 68mm or 73mm wide, almost certainly English thread, and bottom brackets to fit both are also readily available in a variety of types: Square taper, Octalink, Isis or more modern external types .
The rear hub spacing will almost certainly be 135mm, the front 100mm - easily checked with a ruler.
Some useful sites:http://www.parktool.com/blog/repair-helphttp://www.madegood.org/