Surely the real answer is to electrify to Geelong.
Any electrification to Geelong would be along the RRL line, in order to maintain as much separation as possible of Metro and V/line services. PTV's plan is to up the frequency of Werribee line services to deal with overcrowding (and hopefully finally get around to fixing the Altona Loop).
Wyndham Vale is as close as Metro sparks will go, I'd wager. That would involve a new flyover on the Melbourne side of West Werribee Junction and make use of the turnaround tracks that are used for short-starting Wyndham Vale V/line services.
There is another way though...
There's provision for 3 trains per hour as 'short starter' services from Wyndham Vale via RRL in PTV's network development plan. If you get that new flyover built at West Werribee, you could terminate those short starter services at Werribee instead - although you'd then have to completely rebuild Werribee Station with another platform to accomodate that service. No electrification required in the shorter term.
Eventually you could get an electrified 'orbital' service running Southern Cross-Sunshine-Werribee-Footscray - not a full orbital, because it would be essentially a loop around service. But it's possible - just barely. RRL is planned to have 18 trains per hour in peak - 9 to Waurn Ponds/Geelong, 3 to Wyndham Vale, 3 to Ballarat, 3 to Bendigo. Not much room for growth there.
That's going to cost lots of money for a service with exactly the same frequency
as the existing 190 bus though!
Some Siemens trains (with fewer doors) could then have more comfortable seats and a disabled toilet fitted and could run to and from Geelong via Newport, taking pressure off the Velocities.
The Siemens EMU would make a decent template for an electric interurban, yes - but so would a modified Adelaide 4000 class, or the TfNSW V set replacement EMU, or any of the wide variety of off-the-shelf European interurban EMU designs.