Well, something that draws 2w or less shouldn't create a whole lot of heat, and that will vary depending on the DC adapter you use.
But, generally, anything that draws that little power should last you a long, long time. Congratz on the great pickup!
It's always been possible to find good solenoids for cheap on the 'bay. Unfortunately, it generally requires some experience-based knowledge, or a whole lot of reading and searching, or both, to find those deals. It's great to see one of them available in large quantities at decent retail prices. Great score, and I hope it continues to work well, so we can start advocating it to newcomers.