If you wanna talk about the best/quickest return, especially with high development costs, focus on a couple products that appeal to the most people.
- Aero sideskirts: can be used by any 128i/135i regardless of coupe or vert
- Rear diffusor: MANY people want a cheaper alternative to the BMW Performance. If you could come in at the $400-600 price point you would get a lot of feedback.
- Interior trim: everyone loves this, lots of applications, reasonably simple R&D since you're just copying what's already there.
No offense to the vert people at all, but if you're worried about recouping the upfront money OP, stick to products that appeal to a lot of people (i.e. not vert spoilers).
BTW Thanks for doing this!
AR Catless Downpipes | Berk Full Street Exhaust | HPF FMIC | COBB AP | aFe Magnum Stage 2 | E93 M3 Front Sway Bar & Bushings | Michelin Pilot Super Sports | Hardwired Radar | Eurotek BBS LM | Schmiedmann LCI Blacklines | BMW Performance: Spoiler, Diffuser, Shift Boot, E-Brake