By my reckoning, ignoring the electrics and the 158 fleet, but including the 180s (which are unsuitable in reality), to replace Northern's diesel fleet you're looking at 414 vehicles minimum.
However, I'd advocate further- replace the Pacers (including, yes, the three car), 150s and 153s with 2-car 172s:
138 units (276 vehicles)
156s, 155s with three car:
53 units (159 vehicles)
total: 435 vehicles
Plus, for the 158s and 180s, 3-car 170s:
51 sets, 153 vehicles
All with 377-style gangway end cars?
Off the shelf modular designs, of either already-proven or having-bugs-ironed-out-as-we-speak designs, built in Derby, with increased capacity and a fair amount of parts-similarity across the fleets. And when the electrics come along, don't cascade, strengthen. Make with SDO-operation and platform lengthening.
Plus, perhaps, for the electrics- rather than castoffs, a fleet of 379s both for new services and to replace the 332s (I'm sure somewhere else woudl have them) 321 and 323 fleets.
Fantasy stuff of course- insterad it'll continue to be a dumping ground for a good while yet.