I buy my Challenge cars more than I build them. In a perfect world, I could buy the car, set it aside, work on it to make some minor improvements and then take it to the big event. In this perfect world, I may have only driven the car a very few miles (say under 500) before the event.
My luck and personal budget does not run that way. After buying the car, I recommend taking a good hard review of the vehicle to establish a condition baseline. How is the vehicle if the event were today, the day you bought it? What would it need?
I'll take the race Minivan as example. It was bought with a CEL for misfire in #5. It ran poor. It was misfiring. This was a big contributor to it being bought for $1,200. I did put the price of the #5 coil into the budget. I did not put in the price of the #5 spark plug or the other rear spark plugs which I updated while the plenum was off. All the plugs were good and had the race been held "that day" and the van sold the day after, they would have all been fine. However, for me the van was intended to be a longer term purchase. Getting to the back bank of the V6 is a big job that I didn't want to do twice so I attended to the maintenance while the opportunity was there. My intention here was not to make myself a better racer.
I also fixed the LCD screen for the odometer. This ended up being a $12 fix. I did put it in my budget since it was nominal and made for a good story. Had it been more costly, I would have still done it and kept it out of my budget. Why, an odometer doesn't make the van a better racer but it does make it a more desirable item to own than something that would have otherwise been stamped as "True Miles Unknown" hurting the van's resale. Even more than the resale, I put business miles on that van in the 6 months before the event and wanted to be able to track the "tax deducible mileage" accurately. Heck, I put 10,000 business miles on that van before taking it to The Challenge. That's about $5,000 in tax deduction and at 30% is about $1,500 real dollars . Ha
I did fix the AC in that vehicle and kept it out of the budget. Having AC didn't make me a better racer, it made the van a better long term ownership experience.
However, it is know that the '04 and '05 van had inferior timing chain guides (I had a '05.) As such, the '06 and newer seem to sell for more. I looked for a '06 but could not find one in my price range. At the time of purchase, the guides seemed okay. Changing them is a big job and generally makes it a better van. As such, I did not feel that I could reasonably justify making this improvement without a budget hit. Therefore, I di not make that change and went with the existing chain guides.
As luck would have it, in the autox portion on about run #4, the van jumped timing and ran like crap the rest of the event. Finished about 6th from the bottom.
So, after all this writting, what I am saying is I agree with what Tom has wrote and here is how I have managed these "exceptions" in my past Challenge Cars.