If I had a 700 dollar budget, and wanted to buy new, I would forego suspension and maybe gears in order to get quality brakes, wheels, tires etc. The idea is to reduce the number of parts so that what is left is good and not frustrating to ride. Of course, a rigid singlespeed may turn off a beginner, or be unsuitable for your purposes.

So your best bet is to look used since it sounds like you know how to work on bikes.