TBH your car wont get abused doing the TSS. I mean its hardly pushing what the car was made for. If you consider that they can be hooned around on the limits for a full trackday, a few laps of a sprint wont hurt it! Only if you drive into something does it become an issue, and i think most tracks used arent that risky. I ran into a cone and bent the front splitter, but was easily fixed.
Try it in your car and then make a decision as to whether you need another car to do it to your full potential.
Go to the TSS forums and catch up with Phil Cutler and Vix, as they have been competing in a modified mk1 and an SC for a few years now, and they live in norfolk. they will be happy i'm sure to have a chat with you or maybe catch up at an event.
doing a joint venture can often tuen out to be a nightmare. its easy to work out the splitting costs thing for gettign it ready, but only one name can go on teh V5. also if the car breaks down with one person on a sprint, or is crashed by one, then the other person can help but feel like they are missing out. theres no easy way to share that.
check with the current regs for classes, but i would say to have a chance in a std 1600 class all you need is good suspension, bushes, brakes, and tyres.
heres me on a wet sprint!