As a distributor of heat presses, i can tell you there are many different factors into which heat press is right for you.
Style: Clam, Swinger, Draw.
Clam - named that because it opens like a clam. takes up the least amount of space.
Swinger - Requires alot of space to accommodate the swing away upper platten. Heat free work space, however is nice.
Draw - Not a very common style, but lower platen pulls out like a drawer. Heat free workspace as well.
I prefer a clam shell, unless i am doing odd shaped items that need to be threaded on the platen.
Size: I wouldn't recommend anything smaller than a 15x15. Anything smaller may require multiple presses for one design, such as a name and number combo.
Features: non-digital, digital, auto open, manual operation, air operated.
this is all user preference, but i would suggest digital along with auto open.
Price: everyones pockets have different amounts of cash, so you know your budget best. Like moist types of equipment, you get what you pay for.
I personally would recommend a digital, auto open clam shell, 16x16 0r 16x20. Phoenix or Hotronix.