I bought my first car, I personally built my 2nd car (not the cage), I am privileged to have had pros build my third car - I've learned a lot from these three cars. all gp2, MK II VW's
here's what I think:
building a rally car is a major project that requires a lot of mechanical skills and attention to detail. if you have talent in those areas and workspace and time and MONEY build a new car. be honest with yourself - can you build a proper car? there is nothing lamer than poor execution and it will be a big waste of time and money.
if you can do it, I think the main reason to build a new car is you'll have a modern cage in the car and therefore it will be safer.
splurge on the cage, the seats, the harnesses and your personal safety gear.
in terms of performance the key ingredients of a gp 2/ NA 2wd car are:
- the diff and the final drive
- the suspension
- the brakes
- the driver
not in that order... and presumes the obvious: that the car is mechanically sound and reliable.
avoid the temptation to address the powerplant before those areas.
go to rallies and look at lots of cars and take pictures. crew for teams running the Focus. Make friends with professional rally car builders - even if they don't work on your car, their attitude, knowledge and process will rub off on you.