Talon is around on here too.
E30 would probably do fine. The problem you will find with either the E30 or E36 is that nobody supplies wishbones, rear uprights, etc.. So that's all going to be have to modded from the sierra based units or totaly scratch built.
There are one or two BMW builders here.
It's realy up to you which way you go with the chassis build,
Cut yourself ( cheap but long, tedious, prone to errors )
Flat pack ( Still have to weld it up )
Stage 1 chassis ( basic frame built ) ( Tallon or Saturn )
Stage 2 chassis ( suspension brakets added ) ( again tallon or saturn )
stage 3 chasisis ( fully built plated chasssis )
Flat pack is a good option, but if your keen on COST. then you have to factor in the cost of gas and electricity on the welder, and a good build table is going to add a few £'s
I jumped at the chance of a prebuilt chassis when it came up.
Saturn have open days on Saturday mornings ( Dubd by me Saturndays
, all welcome for a chat and a Burger from their washing machine BBQ... It may be worth you having a trip over and having a chat with Spud and Handy.
I am there this saturday, if it was not for me taking the family and stopping over in Whitby I would get you to tag along!
TT
P.S. Do you get your own Tractor? if so what's her name?