First, I'd have to have something vanilla.
I've always quite liked the idea of something like this if I was ever going to select a car with absolutely zero image:
Kia Magentis(/Optima) V6. At one time, the cheapest brand new V6 car in the UK.
Then, I'd remove the majority of the interior trim, boot trim, sound deadening material, aircon stuff, airbags, roof lining and so on. Not sure how much it'd save, but I suspect a reasonable amount of weight would be lost.
Then, I'd drop it on some cheap alloys, give it generic springs and shocks and some half-decent tyres, and a racing seat.
I'd then dress up as an old lady and scare seven shades out of people on B-roads. After I'd got bored of doing that, I'd give it what it deserves at the 24 Hours of Lemons.