Depends on which one you're starting with. Some need more than others. However, as a general list of mods I would do...
-Genuine dial
-Genuine bezel
-Genuine Twinlock crown/tube
-Genuine (78790/A) OR hollow link rep bracelet/clasp
Then, depending on which version you have, it may be a good idea to address the movement (in this case, I'd suggest a 2836 GMT mod with the correct hand stack), and the correct GMT hand (corret in terms of length and arrow shape).
The genuine dial typically runs $250-ish; the bezel runs around the same (at least what my supplier has quoted me). Crown/tube combos can be found on eBay. I recommend fitting genuine parts where possible because gen parts simply look and feel the part (hence resulting in a better rep). All in all, it could be a pricey venture, but will no doubt come in costing less than a gen