Given that the Geelong-Ballarat line is used very regularly for freight you'd think there would be almost zero trackwork required to get a passenger service running, but the real question is what kind of speed would you get? Is the line (both in level of maintenance and in curves etc) capable of running a VLo at top speed?
You might get a decent share of trips away from those who go by car but only if it's faster than (or at least as fast as) going by car. Half-assing it and introducing a slow service because they couldn't be bothered upgrading the track would be a complete waste of time and money.
Would it be particularly difficult to bring the old stations back into service? I know a few are private residences of one sort or other (I think one's a B&B?) but were they sold or just leased? If they have to build stations from scratch it will bump up the price tag considerably, you'd assume.