Essentially it doesn't matter what stock you use, so long as it isn't a slouch and can do 125...
We should just be looking at a new standard EMU and not at anything bi-mode IMO, we can electrify our way out of the lack of LDPE diesel stock, it's harder for commuter stock, but it can be done, would be a lot of wires though.
Electrifying out of the need for new Diesel LDPE stock is a lot easyer than reigonal, reigonal projects at the current rate span into 2030, for LDPE it can be done by 2023 - 2025 without too much difficulty (MML & CRE - CTR, order of 20 390 units)
Listed it many times before, but EC looks after its self, loco haulage north of EDB is easy, and if not, using cascaded 220 units doubled up to EDB/YRK.
GW is taken care of with the 222 fleet, and possibly a few extra 220/1s from ICWC/XC released by electrifiction and ordering them LDPE EMUs (390s for ICWC, somthing new elsewhere)
But then the need to order more 390 units could be over-ridden by HS2, won't need any more Cl390 units if HS2 happens, HS2a opening would proberbly herald the eventual end of the Cl43 formations by cascade of 390s onto 221 routes etc.
Currently set at 2025... Lets say that in this year...
HS2a opens
MML Electification Commissioned
This releases...
27 222 units
20 221 units
These units are plenty enough for the off wires GW area, and continuing through journies on the NWML, the 390s released would be enough to work the WCML diagrams.
'Mainline' WCML services would be run by HS2 CC stock, that will be able to run off wires by loco haulage, as they can be designed to be able to be loco hauled and DVTable, with the loco attaching at Chester (Assuming wires get there) or Crewe, or for other services, Doncaster, York or Edinburgh.
So off wires ECML and WCML services are handled by HS2 CC Stock loco hauled, retiring HSTs from these routes.
The GWML routes will be operated by cascaded stock from the MML, namely the 222 fleet, that can be electrodieselified quite easilly.
The 221 fleet can suplement this, with the same treatment.
Since 2025 is a realistic target for HST retirement, HS2 can provide enough units to do this for the GWML, and the facilities to do it by new stock for the ECML and NW Coast, since HS2 CC Stock can be specified to be able to be loco hauled, and such a loco built by the same designer, also used for Thunderbird duties.
The electrification of the MML would be nessesarry north of the HS2 connection anyway, for running onwards to Leeds and Doncaster/ECML Connection. But should also be completed in isolation for inter-reigonal services on the MML when HS2 takes over the fast services via Derby to Sheffeild, Leeds, York and North. Inter-Reigonal services operated by 5 car EMUs, doubled up when required, or by the same LDPE Stock (Call it the IEP if you like) as the GWML.
So yeah, enough off topic there, and I'm sure somone will read half of this, pick it to peices and say that I'm wrong...
PS: Fully electric services on the GWML will be operated by the new LDPE EMU, this would operate:
GWML
Midland Mainline (Post Electrification and HS2)
Cambridge - London
ECML (Possibly in lieu of 91+nMk4+DVT if needed, that I dowbt with main services being provided by HSCC)