I was able to get an R720 at auction for $70 shipped on eBay, those are the oldest I would go.
If you are going to use it for a game + file server you'd be better served by a newer lower classed model, e.g. R230, R330, R430. RX40 if you can swing it.
I was able to find one cheaper but the price of shipping is really expensive.
R710 is e-waste. Try to get at least like a r730 with ddr4 ram. An r720 is still usable, but getting long in the tooth and only ddr3.
The R710 is a great solution if you find you're not paying enough on your electric bill
Great for the winter time!!!
It's why I still have a plasma TV.
Which I just powered on last week.
Yeah I think if you run something that uses lots of power is fine if its cold outside. You can turn of the heating and get heat from the server rack.