There's a used one for sale for less than 8k thats bigger than your requirements.

if it's the one with the out of state# it's been running in the classifieds for 2 years. it's a come on ad. my neighbor called them last year. supposedly it was pending sale. but they had some newer buildings they'd be happy to sell.
again unless you can live with the doors on either end of the building. you will not get what you want for the advertised price.
i spent 1 year looking at different steel buildings. none of them were what i wanted entry and garage door way.
if you are a do it yourself type and have say 4-6 weekends to spend buy a building kit from sutherlands, 84 lumber or another like minded building supply place.

Infloor heating is the way to go, far more efficient than any other heating for the type of structure.

good idea BUT with colorado's shifting sediment in the long run it may cause more problems then not.
if you were to use the garage as a full time business then i would suggest a good overhead GFA unit. if it's for the casual weekend/project. then a propane heater with fan will heat an insulated garage to a workable temp within 30 min.
you might also consider solar. if i had more allotment space behing my garage i would install a solar water storage tank.