I think you would be much much much better off buying a car already prepared for racing.
the MGCC Trophy series for example has classes including virtually road standard spec. What is your race experience?? Do you already have a licence?? What is your track experience??
It's a dangerous place to be when starting out with no experience, for you, but more so for all the others out on track. Maybe a good idea to do some MGs On Track track days with a proper instructor if you haven't already done so.
why not come up to MG Live in June at Silvestone, it's the MGCC annual event, and it's a race weekend, with many ZRs and some ZS competing in different race category's. Have a chat with some of the race guys and girls (but not just before or after practice / qualy / race/ scrutineering etc. I am sure that they will have time for you. Use it as an information gathering and learning excercise. Several from MGCC racing have gone on to bigger race series ( read much bigger budgets) including BTCC
http://www.mgcc.co.uk/trophy/ex-trophy-stars/
Start small all and slowly and grow as your ability and budget allows
just my advice, as a frustrated racer myself (lacking both talent and budget) so I have marshalled and now photograph it instead (for MG Motor UK, MGCC and TSCC)
even just oust marshalling you will be in a position to learn a lot :cool: