I think you could get quite an amazing mountain bike for $700 range on Craigslist or Pinkbike. Full suspension, if you want it, to be primarily a MTB that you also use for commuting. If you could extend your search to Denver it would probably help.

Just an example - this bike retailed for $1760 new in 2008
https://denver.craigslist.org/bik/4861632767.html

This bike was $1650 in 2012. It's full suspension but its components are a few levels lower
https://denver.craigslist.org/bik/4836517642.html

You can research older model bikes on bikepedia.com

For commuting you can run on the MTB tires which is a bit slower and wears them down, or you can swap out the tires for slicks, or you can keep another set of wheels with slicks for a quicker swap.