Set-up fees and monthly payment using Pro Sites and Gravity Forms User Registration

Hi folks

I'd like to know if (and how ) I can set up this scenario using Pro Sites.

The client has 3 choices. Each choice is made of 2 parts that play together:
- one for the set-up fees (one time payment)
- one each month

For example :
- choice A = 100 € for the set-up(set-up fees) then 5 € per month
- choice B = 200 € for the set-up then 10 € per month
- choice C = 300 € for the set-up then 15 € per month

The client chooses one template, then just after that he needs to choose A, B or C, then just after that he needs to pay 100, 200 or 300 €. Then he sends me some stuff I'll set-up for him (that's the reason for the set-up fees).

For the moment I'm using Pro Sites with Gravity Forms User Registration add-on.

NB: The client can change his monthly offer when he wants to (upgrade or downgrade).

Considering all this, is this possible to set-up this scenario? Any advice?