First A class car you should go for is the short ratio tarmac T3 NSX or Ferrari, details on the differences between the 2 can be seen in
This discussion thread here. Takes about 266 points to fully equip it excluding safety upgrades. After that you may either finish up the cosworth or create a new dirt NSX.
You may quit races and with for 15min if the track isnt viable for any of your cars. The short ratio NSX is viable on most tarmac tracks, you can check out
RD's racing guide for more details on tarmac vs dirt.
I have found the cosworth to be rather effective at earning points on parkland as well, not just mudpit, especially at lower RS level. Stone park however is not as good