Recommendation to upgrade from 11.5.5 to R12

 Hi ,

Client is on 11.5.5 and have done lot of customizations and would like to have  a phased upgrade to R12. I wanted to know what are the recommendations or considerations that should be looked at before making the desicion.

1)Should they take a big bang approach and move to R12 directly ?

2)Should they move few modules say finacials to R12 and leave the rest to 11.5.5 ? What are pros/cons ? What is possible?

3)Should they move some modules to 11.5.10 CU2 and some modules to R12 ? If so what are the modules that can be moved safely to R12.

4)Should they move to first completely to 11.5.10 CU2 and then move few modules to R12 ? If so what modules can safely be moved to R12 ?