Did almost exactly this a couple weeks ago: Replaced My Existing Security Panel with Konnected Security - My Writeup - #27 by vseven
As for the wiring I used the standard plugin USB adapter the tablet came with at the panel…the easiest route if you have a outlet there. You could also use a standard car to USB adapter if you already have one lying around. Plus your old keypad wiring should be fine carrying 5v @ 2a through the wall.