Its a new day so lets agree the E30 is a great car and its cool people have differnet ways of making them go fast
Just to clarify on the P8 bits etc...
Project 8 Racing is a company that holds the MSA permit to go racing. P8R then runs 2 race series the Production BMW Cup which is based on E30 318i and 320i and Racing Saloons which is mixed make pre 93 saloon cars, more similar to tintops etc.
In PBMW Cars built very much to OEM spec (120bhp 318i-140bhp 320i). The main custom component on these cars is the dampers/springs which are free based on 1 way adjustment. Roll cages, adjustable top mounts, bushes (non solid) are allowed. there are more changes you can make but its basically a case of blue printing a BMW car and letting driver tallent show. The winning drivers here are very quick, for example this years winner is Ben Winrow winner of the Clio Cup, Fiat Arbarth 500 etc.. feel free to look him up but he wouldnt be out classed in BTCC, previous winners such as Rob Smith are racing at the ring in modern M3's etc..
Entries.. something like 100 drivers per year, so nothing to do with me I can hand on heart say its serious racing and its far from easy. Cost £5-15k per anum I would have thought. Cars likewise £5-15k to build.
RS is a lot more open, its class based pre 93 saloon cars of lots of makes, some drivers just go because its a bit of fun to run their favorite car around racing with whatever is near them rather than considering out right winds. Others are people who are very quick link John but dont really want to empty many thousands down the drain in some other series. Still running at the front in the more competitive classes could easily see 10's of thousands spent per year.
Re Test days, yes you need a race license, your car needs to be able pass scruitineering etc.. There are no real rules as such as there are on track days and timing is very much part of it.
If you want to get an idea of the pace of either series, look at TSL timming at say Brands hatch Indy and you will get an idea of what times people can put in. A well driven PBMW car is down in the 58's, TTRS are very varied but I expect my car will be capable of a 56, although maybe not with me driving !
Re engines, pretty ,much all race series are based on a standard block to start with, if not they are somthing like sport Libre which is real cheque book racing like you have never seen. I actually came to the E30 after several lotus some of which had non standard engines which whilst fun are not of much use to go racing with.