It really comes down to what software you ultimately want to run (335is or M3/GTS), power goals, and your budget IMO. I'm currently running M3 GTS software with the M3 drivetrain (axles, DS, and diff w/ custom 2.81 gearing). This is probably the upper end dollar wise but gave me everything that I wanted this car to be.
Staying with 335is software will limit you to 335is gearing and drivetrain for the most part. You could use a 215mm 2.81 FD from a 335d and it will work. Probably the cheapest solution. You could move up to the M3 drivetrain with a 210mm 2.81 FD from 80's V8 BMW's and retain the M3 LSD and diff, axles, and DS (needs to be shortened) and substantially increase the drivetrain strength in the process. This would also work with the 335is software.
The M3 software will require the M3 GWS, center console pieces, switches, etc., some repinning of connectors, flashing, coding. You'd need the M3 drivetrain and the M3 diff could be geared from 2.81 to 3.23 with the available R&P's out there, but it's not a cheap conversion. You would get all the benefits of the M3 software though, faster shifts, adjustable launch control, no need to hold the brake while stopped.
This is just a quick overview of what I picked up while researching for my car. If you have any specific questions I'd be happy to help if I can.