in my view i would build something to alrc spec, firstly because you can enter an event with any club then and secondly the cost of the event. as for the car, personally i would go for an 88 or even 93. if you want to win trophies on a budget keep it a 100inch and go for class wins. but most importantly what ever you decide on make sure it is reliable, i'm sure you have heard the old saying "if you want to finish first, first you have to finish" you could be leading and something silly could go wrong. i did my first comp on monday in my 88", we were in about 8th in class after 2 runs and on the 4th the a bearing went in the engine (i think the engine was quite worn anyway) but still it is quite annoying but thats racing i suppose.
but good luck!