Look up BWM E30 racing in Australia, there a one make serices based on the 325i
HERE IS A LINK
www.e30racing.com.au
THey are NOT fully race modified but they are prepared within their rules, they need to use standard components like inlets, heads, compression and factory computer. (but they have been known to cheat
)
They have about 200 hp at the flywheel
Some people have mod'ed and they pull about 260 hp with thottle bodies, computers, cams. I think there is a conversion using the diesel crank that makes them about 2.7 litres, no more power, but better average torque. This is fairly cost effective as a lot of the parts are available
If you go all out (and I know one bloke who has) they will give about 110 per litre in race trim (ie reliable) I think he is about 2.9 litres with custom crank, custom everything, actually!
Of course the best E30 BMW to use is also 2.5 litres, but it only has 4 cyl...
A full house XE motor will get the same power as the BMW from 2 litres (320 hp), and you will be effectively looking at a BTCC motor, but running to about 9500 rpm. because of the revs it will cost MEGA and rebuilds will be common because you will be looking a lightweight forged slipper pistons with really skinny rings, steel crank and rods, biggest valves you can get, titanium top end a cams up over 300 degrees. Oh and you will be looking at dry sumping as well... so that would be about 12-4k GBP (I am guessing you are in the UK)
If you keep the XE to reasonable power you can buy upgrade kits, around 200 only requires a computer, exhaust and throttle bodies
(bigger cams require new pistons, then you can push to about 240 before the rods need doing, then to about 270 is the limit for the crank)
Then it comes to the chassis... an Astra is going to have a serious weight advantage over everything else, the cavalier will also have an advantage. But the BM has rear drive so is asking less from each of the tyres