I'd echo what Cheetah_SA said about flying into Siem Reap. Various airlines (eg. Vietnam Airlines, Bangkok Air) fly in there; Phnom Penh is optional, though I found it a fairly interesting place to visit (interesting in a very different way from Angkor & Siem Reap), but I would fly in & out of Siem Reap.

I would guess that 2-3 days would be enough for most visitors, though the Angkor complex is huge and if you wanted to cover it thoroughly, you would definitely need more -- some temples are much more remote.

I haven't been there in May, but I'm guessing that it will indeed be HOT. Drink lots of water.