defintely the Unifi APs are meant to be ceiling mounted.
i'd maybe avoid the loft though and just put it on the upstairs landing ceiling.
the AP AC-Lite is the base model AP, but it still performs well, and "should" be enough for a normal house coverage. there are lots of other models though that up the performance at each step.
whatever PoE device you end up with, make sure it matches the PoE version that the other device support (see previous post regarding 24v PoE and 802.3af.
also, for PoE, make sure you invest in some good quality cabling (solid core, shielded etc).