At NHRA.com, they have the basic's and classes listed. You'll have to do a bit of reading to do and understand the wide ranges available.
Get the car in as a complete condition as possible. Since your looking at a drag only car, things may be missing that have no effect on the car except weight loss. Purchasing someone else's project or light strip car is a good option. I know what you said above but there is an advantage to this.
First it will save you money, time and busted knuckles.
Second, you'll be going over the car with a fine tooth comb before it hits the track. Be it a dead stock car or mildly modified. You'll need to do the same work either way. So getting a hot rod/race car should NOT be out of the picture.
No matter what engine is in the car, go directly to the strip and start racing it as is. (After you go through the car and right the wrongs of course.)
The rule Book is available on line or in paper back. Read between the lines of what is written. There's wiggle room everywhere! The MoPar engines and chassis book are excellent sources for racing. You should grab these books.
Once you start running down the track, you'll start making mods to the car to increase your speed and lower your times one step at a time. Enjoy the ride & journey.