if it's easier for you to modify the front of the already existing M5 then probably we can have the 535 sooner. Do you already have it? Unless its easier for you to create the 535 from scratch. The most accurate 3d model I found (it's not that great but it's the closest compared to the other 3d models) is at :http://img-new.cgtrader.com/items/29357/large_bmw_5_series_touring_e34_1993_3d_model_3ds_fbx_c4d_lwo_lw_lws_ma_mb_obj_max__812b1ad1-07d6-484c-b493-cfa9f34bf19e.jpg. Here's a front of a 540i I find very accurate
cs1.gtavicecity.ru/screenshots/9a0d4/2014-11/original/ec718dcf8c38e077d32e5187e4ee7bcef38a34c3/228626-6.jpg
Headlights of the car in the pic are detailed whereas in the M5 in my screenshot above (in city car driving 1.4), the details are HARDLY visible. Anyway changing the whole front and adding the detailed headlights, the 535i grill and everything should make it look like an E34.
If you'd still rather do it from scratch let me know.