At the time I was also annoyed to find out that with the remaining 6ish weeks of the season, there weren't any D-class races I could enter without buying tracks to get the min. participation for a C license, so I was stuck at D no matter my safety rating. I think the 1 week SRF ran on a track I had, I was out of town or something.
Did you own a C-class car? If not, who cares if you have a C-license? If you did, then you can race class C with a D4.0 license even if you never step foot in a class D car.
FYI, there will be 5 series this season running base content cars.
D:
Global Challenge Fixed (CTS-V.R) w/ 4 base tracks
Grand Touring Cup (Solstice) w/ 4 base tracks
Spec Racer Ford w/ 4 base tracks
C:
Advanced MX-5 Cup
Global Challenge Open (CTS-V.R) w/ 4 base tracks
Since you brought up the idea of $60 of content, you can definitely get more than one series for that. Especially with four series running free cars and 4/12 weeks on free tracks. If you buy the following tracks:
Donington
Sebring
Mid-Ohio
Watkins Glen
Mostport
Those 5 tracks will give you 8 weeks of racing in each series. Then you could either buy another track (CotA, Bathurst, Interlagos, and Oulton Park get used twice by these series and are all built to the latest standards of quality) to make it a 6-pack, or the Ruf. If you buy the Ruf, you get three more series to race with the tracks for 4 or 5 weeks. Either way, you've got non-Rookie content to race every week, and enough to earn participation credits. Yes, you'll probably need to buy more next season to keep 8 or more in those series, but a 3-pack should accomplish that easily.