There are 2 classes - one class for 1974-1980 cars using 2 litre engines, one for 1971-1973 cars using 1600cc engines (mostly Lotus Twincams).
You can find out more at
www.classicf3.co.uk
As you say, the smaller engined cars can be bought quite cheaply at the moment, and that class seems to be picking up in numbers again this year. F3s are also surprisingly cheap to run.