Make sure you get bikes the correct size. Although you can ride bikes that are too big or small, you will be much more comfortable and confident on one the correct size. If you have to chose between 2 sizes, go for the smaller one.
When chosing a frame, look for some threaded eyelets for rear luggager rack and fenders. You can find them just above where the rear axle fits into the frame and at the top of the seat-stay tube.
If you are looking at used bikes, go for a known major brand (Trek, Giant, Specialized etc).
Make sure that the heels and pedal cranks are smothe, not wobbly.
Check the transmission for wear, if the cogs have a shark's-tooth profile they made need replacing.
For new bikes, non suspension MTBs of acceptable quality are not very common.
See
Jamis