Look on CL and Ebay. Your looking for late 90's steel frame with Rock Shox Indy fork and 7/8 spd Grip Shifters.
Under $300 easy.
1. Steel frames of the day had some give to them. This means a smoother ride. Not to heavy. Brand name???? Who cares.
2. Indy fork is dead simple and takes the edge of the trail. Take the caps off, grease and go. Simple
3. Grip shifters are dead simple. Twist and they shift. Most even had gear #'s on them.
The only new things you should have to buy is a chain, tubes and tires for your terrain/area.
Saddles depend on your butt. You have to try a bunch yourself.Find a shop that has a saddle seat that you can try right there in the store. Most shops have these things.
Check Park Tools site and Youtube for maintenance. Gain experience here. Real quick on chains. Cross Country Wet lube okay.
Check tires every ride. Play with the pressure to see what works best for you and where your riding.
Water. Drink plenty of it while your riding.
Sunscreen. Go back to youtube. Find sunscreen song. Play it and believe it.
Have fun and don't take yourself to serious. Don't forget to try and find different ways of getting off your bike (fall).
I've been riding mine for 11 years and enjoy every 3/4 hour ride. Colorado is a nice place for a mountain bike ride by the way.