Have you raced before?
Do you know someone that has raced for a long period of time?
If you have not raced before, heck even if you have then I would suggest buying a race ready car. Normally you will be able to buy a car a lot cheaper than it will cost you to build it. Plus you will be able to race sooner and be competitive faster.
People told me to buy a car already built and I was totally against it, now I regret it. When I do it again I will buy a race ready car.
The cars are much faster than they look from the stands, and driving is much harder than it looks. (even in the lower classes)
If you know somebody that has raced in a higher class for a long time then building the car shouldn't be a problem.
If you are thinking about buying a race ready car there are article on "how to shop for a racecar". Or have a veteran from MSTRC tell you what to look for when shopping for a racecar.
Whatever you end up doing you will have a blast and good luck!!!
P.S. My opinions are formed from experience