Southeastern would ideally not want another fleet that has nothing in common with the rest of the fleet, given the extra cost of training staff and potentially adjusting DOO kit and platforms to accommodate them. This would rule out 350/1s, 458s and anything else not already at Southeastern. The only stock that might help is more 377s, but this would mean Southern need something else to replace them.
The reality is though that Southeastern have been subject to a poor environment for ordering new stock: first the constant short-term extensions of the Govia franchise prevented an environment in which a new fleet investment was worthwhile, then a combination of post-covid uncertainty and a DfT procurement setup that makes snails look like cheetahs has meant a new fleet still hasn't been approved. Had Govia had a long-term extension in 2018, a Networker replacement would already be in full service, or had the DfT been able to sign off the current tender in 2022, when it was put forward, the first units could be beginning testing with a view to being in service fully by the end of next year.
I wonder if it's worth considering setting up a proper procurement for bespoke parts for the units left, even if fairly expensive? They are sort of already doing this with the C4 overhauls the BREL units are about to receive, but if no other fleet alternatives exist for rest of the decade at least, they are not going to be able to rely on part-stripping forever.