The 300P has dropped some more to just under $70.... Now I'm wishing I'd save the $40 and went that route over the 400P (there's very little difference in output).
For seating tires you're going to need a tank for sure. One of the small 2-gallon portable ones would probably be enough but you need a "blast" of air to seat a tire. You'll also need to pull the valve core and have some kind of air chuck that is open without pushing in on the valve and a separate valve on the tank. I've got an old water type fire extinguisher that works well for that purpose.
I picked up a pair of these off amazon to replace the one on the 400P I just bought. The angled air chuck wouldn't work well with my valve stems (bad angle) and really the deflator option was just kind of silly.