Andy, I joined earlier this year for what its worth heres my view because i would do it differently if i started again:
I wanted to try all the cars straight away and try to get my license then the next one and the next one so i forgot to have fun and was concentrating on SR all the time. Got into the Star Mazda and did a few races then back in the MX5, tried the spec thing. If I stated again I would do a few MX5 races then get the Skippy. Buy the tracks required for a season and concentrate solely on that series and stuck with the D license.
Instead I raced to a B License, dabbled with all sorts of cars and didnt spend enough time practising. I'm not sure but I think the higher the license you have, the harder it is to increase your IR and SR, but a lot easier to lose it - (I got hit twice on Friday and lost 0.49 SR).
So my advice, get your D License, race the Skip Barber for a season and remember to enjoy it.