Standard Sierra hubs were 4x108 there are loads of alloys on auction sites that will fit but watch the offset. Too much and you'll impede the arches.
Most people go for 195 width and tyre pressures about 19-20 psi.
As for parts, keep a look on eBay as they sometimes come up. There was a chassis and weather gear for less than £200 last week.
The s7 and Exmo are both monocoque design so parts may cross over except for front suspension. The chassis is quite wide, I've got cobra roadster 7's in mine but mx5 seats should fit.
If you are going to register it you will need to put it through the iva if it hasn't been registered before, it's not impossible to do on an older kit and it's been done recently so the information is on here somewhere.
Welcome to the madhouse
Dave