At 1420 MHz, that's about 2 cm of allowable surface error. Look for something which approximates a smooth parabolic curve. If panels are missing or bent, performance is going to suffer. Next, look at the mounting hardware. If it's rusted, you're going to have trouble getting the dish apart, and more trouble reassembling it.
Weight is sometimes a consideration, as is wind loading.
If these are concerns to you, a mesh dish may prove more realistic than a solid one. Many of the accessories which come along with a satellite TV dish will be of limited use for SETI, and therefore you should not pay extra for them. TVRO receivers are great sources of microwave components, but unless other civilizations utilize exactly the same TV transmission standards we do, they're not particularly useful as SETI receivers. In the final analysis, your budget will likely be your chief limitation, so go with what you can afford. Additional information on various SETI antenna options, along with vendor links, may be found in the Antennas and Feedhorns chapter of The SETI League Technical Manual.