is it legal - IVA question

There's been a few on here iirc, one mx5 springs to mind.

The body you put on top will need to pass the IVA regs, the main issues are no sharp (less than 2.5mm radii) corners on the body or trim (both inside and out) and glass which must be marked with a recognised standard.

Most of the rest of it should be covered if you submit with all the standard BMW parts fitted.

This is the book they issue as guidance for M1 (less than 8 passenger) vehicles. It's a bit of a tomb but not to bad when you get into it.

assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/930247/individual-vehicle-approval-inspection-manual-passenger-vehicles.pdf

There's no requirement for airbags etc. in the IVA - I've just gone through it with my kitcar. The rules aren't particularly bad but don't always make the most sense.

In short, as above, 100% doable, as long as you meet the requirements and your welds don't look like they've been done by a blindfolded monkey