Thanks guys for your feedback. It's really appreciated.
Regarding rear axle reinforcement, I've got the Eyeball engineering torque bar.
http://eyeball-engineering.net/TorqueBar.html
Intrax answered to my questions. They can build specific coilovers based on 944 ones on my desires. Nevertheless the price is too HUGE for me (2350€). A bilstein cup set is about $2000.
UK Pro-Tech coilovers can also be a good alternative regarding the price of an equivalent set. Especially if it possible to select coil overs with off-set lower eye mounts.