Girona Airport is a 35 minutes drive from Mas Prim. Both Ryanair (all year) and Transavia (from April until October) have cheap flights to Girona. There’s plenty of affordable car rental options at Girona airport.
The only road to Mas Prim is an adventurous 3 km’s dirt road. You won’t need a 4×4, but the road is quite bumpy at some points so you’ll need a car with normal clearance: make sure your car is not too low.
There’s a direct bus to Barcelona from Serinyà. It’s the best way, in our opinion, to get to the city!