First, don't build a car and don't start with 4wd. You can buy an already prepaired rally car for as little as $4k, it will cost you $15-20k to build one. Start with something slowish, like a VW Golf (excellent starter car) Don't worry about how nice a car you have because you ARE going to crash. Take the money you save, and put it towards the school or entering rallies. It costs more to do a rally than you think. There are a lot of expenses that you can forget about, such as hotel rooms, not just for you but the crew of friends that you will need to do the service, there is food for every one, also you will need a vehicle to tow your race car as well as a trailer, gas for both race car and tow vehicle, entry fees, tires, spare parts,etc. Navigating sounds better already. You will also learn a lot about driving by navigating. Just getting the opportunity to watch someone that knows what they are doing will teach you a lot, but the school will be most benificial.
Check this out: www.specialstage.com it is a rally forum, you can post there that you are new and looking for someone in your area to navigate with.
Also check out: http://www.cwl.com/bensrallypage/forsale/forsale.html
Used rally cars, this will give you an idea what's out there and how much