BrainOfCeleron

SeaSonic G-series 850W or X-series 860W

i'd say a low end seasonic unit, or dumpster dive a power supply out of an HP business workstation, those are rock solid.
(since it looks like you're salvaging already :p)

because if you dont need much storage a 128GB ssd is around the same price of a decent hard drive, and MUCH faster.
 
even on a crappy netbooks SSDs are a considerably boost in performance.

Your power consumption definitely won't breach 100W so have no worries about wattage. I'd suggest going to look for an old Dell or HP PSU (depends on what model but a lot of them have nice Delta units) if possible or a 300W Seasonic unit.

Personally, I'd just stick to the APU for a while and upgrade to a discreet CPU and GPU setup later. If you add a GPU, you'll just essentially be throwing away the use of the graphics cores on your APU. Maybe toss a few dollars at your case to get it some more or better fans if you're worried about temps.