I seriously considered doing a scratch build but I figured that in the end I wasn't actually going to be saving enough for it to be worth while, especially considering the time it would take and chance of giving up.
I was admittedly going to cheat significantly and take the route KVA, KCC and others did and take the suspension setup/brakes/steering off a donor car (an RX-8 in my case), copy the mount point locations to either a space frame or aluminium honeycomb monocoque and start from there.
I bought a welder and started practising. I bought a chop-off saw, I bought books on suspension and building your own cars. I got as far as a CAD model with a few parts bought, spent ages looking through the build logs (for example) the GT90 scratch build and other scratch builders and realised that I was probably looking at 5-10 years *just* to do the chassis/bodywork and *then* I'd have to do the rest! While I could make some savings costs on a space frame the bodywould would need a substantial amount of extras in plywood, foam, filler, fibreglass (for the mold) etc.
Very simply I couldn't make a set of bodywork from scratch as cheaply as they can be bought and the chassis was only going to be a smidge cheaper.
At which point I discovered GTS Tuning's offering and after obtaining permission

decided to go ahead and the rest is history.
I'm still likely to be trawling ebay for bits (and will see which of the RX-8 bits I haven't yet sold fit[1]), will be making a number of bits rather than buy but in the end I see it as a trade off between cost and time, spend money or spend time, which one you spend is up to you.
Good luck to anyone that attempts a scratch build, at some point I might tidy up my RX-8 based chassis plans[2] to fit the GTs40 bodywork and post them but I wouldn't hold your breath!
D
[1] Things like the 323mm brake discs for example.
[2] While waiting for stuff to arrive I also created a model of the GT Forte GTs40 chassis but quite frankly as I've paid for the real physical thing and the plans aren't unreasonably priced I'm not sharing it with anyone who hasn't also bought one.