Things to avoid:
- Suntour groupset - yes I know some of you swear by it. But Shimano has wide compatibility and wide interchageability and endless possibilities for mix and match. And Shimano's index shifting just works better with no fuss.
- 126mm or 130mm rear drop out spacing. Yes- some of the older frames had 126mm drop out spacing. Look for something from the early 90's and on, with 135mm spacing. Reason is there are tons of 26" 135mm wheels on the used market. There are very few 130mm ones and no 126mm ones on the used market.
- Freewheel hubs. Early MTBs sometimes came with freewheel hubs. Avoid. If you get one of these you'll need to eventually swap out to a cassette hub wheel. And most freewheel hub MTB's came with 126 or 130mm spacing (see above), making the swap nearly impossible.