So based on my research I can tell you the following:
- uprated brakes
/// pads you can do whatever you want, if you change calipers/discs you need papers. If you can get better calipers/discs from same model that fits right on the same brackets, it should be OK as it's OEM. Changing discs means different dimensions or changing type (from standard to drilled or slotted)
- 3.5cm-5cm suspension raise (found a kit already for both)
/// forget about it without expensive DTC process
- EGR + fuel tank + diff bash plates (car already has an engine skidplate from stock)
/// should not be an issue
- snorkel
/// should not be an issue
- smaller steel wheels, with bigger tires (total diameter should remain "roughly" the same)
/// wheels are quite easy to do, you get whatever fits, and you just make sure to get papers and arrange an MFK visit. If you get wheels directly from CH it's more straightforward as the seller will hook you up. If you get wheels from abroad, you'll need to find the brand seller here to get you papers (at a cost of 150-250 CHF). I think there are a couple of shops that can give papers for "any" wheels you can get, but I haven't tried it myself. I'd stick with a known brand and try to source locally TBH.
- strip interior (MAYBE some bucket seats, lets see)
/// as long as seat belts and original airbags are in place, you are OK. I took one of my cars with a bucket seat through in September, no issues since the car did not have an airbag in the original seat.
- rollcage? (maybe - depends on rules, tbd)
/// if you go for full cage, I would very strongly suggest to forget road usage. I would not drive a fully caged car without a helmet - it's just too dangerous. Also I don't even know if you can get a DTC approval for full cage on road.
Best bet is get a garage that does race/rally preps and do everything with them. They can guide you through the process. If you're a DIY guy, it's going to be more difficult to manage the admin side.
If you don't mind me asking, what car and what rally are you talking about? I'm thinking my options for some offroad/rally-raid stuff this year on a very amateur level, but with next steps later on in mind.