I would reccomend buy first - And even though it does sound like you have plenty of resources to build a good car - buying has some advantages.
My experience was trying to build an RX7 in 2000. I got about $3500 into it (donor car/suspension) before I ran into trouble finding a cage builder (they were out there but busy and cages were in the 3-5K range - if they had time at all to build one). Not too mention the donor car had issues of its own. At that point I sold it and bought the turbo colt. It still needed about a $1000 bucks work to get it rally teched but the overall cost was much lower and the car was properly built (at the time).
Point being is sometimes the build gets stalled or the project takes a lot longer than you think. The amount of half built rally cars is most likely staggering.
Secondly if you buy a starter car (fwd non turbo) and you really get into the sport - its more likely you will want to jump classes and get into awd/turbo - which you could build yourself to your liking. Then you are not stuck with a 20K focus that you are no longer interested in driving at events.
Plus you can use your starter car during the process of the other car's build, and sell it when you are ready, or use parts - seats etc for the new car.
The main problem for rally for normal enthusiasts is money - just updating a car for next years rules can be costly, plus event entry fees, unforseen problems, etc. A starter car can get you a decent ride at someone elses cost and you can estimate what you'll need for a future car.
Just my opinion here.