I don't know that the extension of east link will effect Frankston North prices too much. You can nearly get on East Link towards the city from Seaford Rd, so I don't know how the new extension will help commuters to the city from Frankston North.
As for Baccus Marsh, I'm not a fan of it from an investment point of view, though I don't know much about it.
Sunshine West I don't know how many you can get for under 300k, but if you can get a decent house there, that could be a winner.
I follow the Frankston North market very closely. I'm finding at the moment that rents are stablising there at the moment. 6 months ago properties were getting 250p/w now lots of properties that are advertised at 250p/w are dropping to 240p/w. Frankston North is generally an area where investors go, rather than owner/occupiers. So I think in the short term prices in Frankston North will stablise (like they have for the last couple of months). Don't know how long it will stay stable for though as in May last year prices sky rocketed out of no where. Stock is quite low at the moment so be interesting what happens in the next couple of months.