Just to work that out, that would mean a lot of Pacers and quite a few 150s, which could either be transferred out or stay in Wales. This is a back-of-an-envelope consideration. I'm assuming all Pacers are dedicated to the Valleys
1 X 121 "Bubble-car", possibly keep.
15 X 142s, scrap (hopefully)
15 X 143s, transfer out (to Northern, along with FGWs units as a common pool)
30?[Please confirm] X 150s, keep 12 within franchise to back up the 153s, transfer rest to FGW (to replace all their 142s and 143s, sending their 142s for scrap)
Therefore, you give Northern some extra unit capacity, get rid of all the FGW Pacers and reinforce Welsh local services, on top of all the electric units you bring in. With their higher availability, 60 should be enough for a like-for-like replacement, with 75 if you want an increased service frequency.