To be honest, I really like this idea, but there should be several restrictions about that, like:
- Player shouldn't be able to purchase maybe like more than 3 cars since they will just buy bullets/infernus/NRG for low prices and fill up all garages, and if we add that you can sell vehicles to other players it will be just pure easy money
- 1 day a month seems legit, but it should be 1 in game day as well, Friday (as you said).
- Ammo restriction as well
Overall, +1