Frankly, unless you happen to know the seller and his machine, I suggest you go new. This way you get a warranty, and often the right to try and return for refund or return for an upgrade. Also, the major suppliers always have sales and open box specials; so you should do just fine on a new machine, albeit with a paucity of choice with your budget.
Your budget places you at the mid to high end of single boiler/single boiler-thermobolck market, at very lower end of the HX market, and well out of the double boiler market. At your price point it might be tempting to go for a single boiler/thermoblock machine with all the bells and whistles. Having been there repeatedly I suggest simplicity is better. So I suggest an HX. In any case, all HX machines work very well.
HX machines are all more or less the same in principle, but different in execution. One major difference is in PID vs non-PID; I thought I wanted a PID, but my current machine has no PID. Another is the group; you'll find fans of both the ubiquitous e61 and proprietary groups. Honestly, I don't think it really matters, as at your price point you won't have a great deal of choice.
I have a Magister Stella, which is a good example of what one can get at the low end of the HX market. I noticed a new seller on Amazon flogging "used-like new-full warranty" (demo units I wonder?) in the $500 to $700 range. New, they run about $1600 from commercial equipment suppliers, as they are rated for commercial use. I think Parts Guru http://www.shop.partsguru.com/main.sc
is the US distributor.