Under most circumstances I would say pay cash, or as suggested above, buy it as you build it.
However, as was also pointed out ... life is short so don't wait too long.
Another way to look at it would be how much money do you want to have tied up in rolling stock (cars). Maybe have a more modest daily driver and put the difference into the Cobra fund.
Also look for a car that's already built, you can save a lot of money that way too. Downside is, if you really have your heart set on an ERA 289, there are not that many cars that come up for sale ... but eventually they will, you just need to be patient and jump on it when you see the right one.
Best of luck and don't give up!