Hi 888z and congratulations!
Some things to be aware of for starters:
Some early models of the 912 were vulnerable to crankcase fretting ($$$$$).
Early 912s have a 1500hrs TBO, later models have a 2000hr TBO.
Later 912s have a much improved ignition retard for starting. Poor starting on earlier 912 could lead to sprag clutch damage ($$$).
So, I'd be looking for a 2000hr TBO engine, recent enough to have the new ignition soft start setup.
Also, the 912 has a 5year rubber replacement requirement, including the fuel pump and all the engine hoses. There is some cost in this, so when is it due?
Rotax have an excellent website with a user's forum where you can read up on all this.
Be aware also that the Savannah has comprehensive 1000hr checks to be carried out on the airframe. Again there is some cost, so when is that due?
Apart from that, I would be looking at the general condition of the aircraft, how it has been cared for. And I would be going through the maintenance records to see that it has been kept maintained and if there is any damage history.
I would most strongly recommend that you get an experienced aviator and/or maintanance person to check over the aircraft and paperwork with you.
As for VG vs S model:
Most VGs will have the older engines: (see above)
The S is definitely more roomy and that is an improvement. (It also has more baggage space if it has the extended baggage mod, though baggage is still limited to 20Kg).
The VG probably has the edge on the S when it comes to STOL, though the S is still very good.
The S is a bit faster in the cruise, though we're not sure why.
Also, these are lightly constructed aircraft and will have a shorter working life than older GA aircraft.
So, generally speaking, I would go for the S unless your main aim is to win STOL comps.
Finally, 80 vs 100hp:
I cannot comment on fuel consumption. But I think you will appreciate the additional performance of the 100hp engine, and especially if you will be doing STOL work and/or will be flying with a passenger (loading makes a big difference).
Happy hunting and let us know how you go!