So, not sure how best to ask this, hopefully this is the right area to post this thread. Longtime lurker, first-time poster. First car was a 92 E36, haven't owned a bimmer in a while.

A friend has a 2005 Z4 3.0 auto. It's a salvage title and he's been driving it happily for the past few years, up until a few months ago. The airbag light came on, diagnosed as an issue with a module. We have a good local indie BMW shop, but after $1000+ in parts and diagnostics, they weren't able to solve the original issue. Between all the removing and reinstalling modules, the inspection expired and the car started having issues starting. We assume the starting issue is because something wasn't reinstalled correctly, and at that point my friend decided to park the car for the time being. He ended up buying an 04 330xi to daily, and then someone ran into the Z4 on the street. A fender being crushed was the only real significant damage, but after the insurance payout, my friend is thinking it's time to cut his losses and ditch the Z4 instead of continuing to invest money into diagnosing and trying to fix the original issue.

How crazy would it be to try and death kart it? The engine and trans were completely fine the entire time he owned it. Up until the electrical issues started, the worst things about the car were a broken motor for the top, and some mild body damage and other misc issues that didn't affect using it. So if I could flash the ECU or use a standalone that cuts out the need to have all the other modules present, and I don't need the airbags, radio, hvac, etc, how doable is it to turn this into a stupid, light track car?

I'd be adding a roll cage, removing all the body panels and interior, getting some light racing seats with harnesses, etc. This would only see autox and some light track duty, nothing where real safety is a concern. My last BMW was an E36, so I'm not up to speed on this gen's electronics and what's necessary to do something like this. I'm ok spending a little money, but I'm hoping to do this project for under a few thousand if possible, otherwise it's probably not worth it and he should junk the car or sell it for parts.

I've replaced an engine or two, done tons of suspension, brake, and mild engine work, but haven't done much with advanced electronics.
Any and all advice would be appreciated!