Be sure to check this out to get the biggest bang for your buck. Great choice in the vintage F1, the GTR 3 and the Porsche 911 GTS Cup are my personal favorites but be sure to Test Drive as you can try any car free of charge to determine which ones suit your tastes.
As David says, you can test drive any car through the in-game store so try the cars for yourself. "Must-have" is based on what you enjoy, not us. We rotate the classes we use in the club regularly so there is no club-specific content you need. The GTR 3 & 4 tend to be quite popular, as do the DTM `92s and NSUs. But again, try before you buy because there's no sense buying cars you don't enjoy driving.
Who know what will happen after next weekend, some cars maybe get their few weeks.
EDIT: As for suggestions, take packs. And all that David said. I started with free stuff, learning tracks and then when I joined racing club, I took some that were missing from starter pack. Mostly GT3/4 and WTCC/TCR. What is biggest + for this game and why I only play it of all sims, there is something for anyone.
Group 5, Nordschleife and Bilsterberg Grab just what you need, id expect sale relatively soon, long time since we had one For online, GT3, 4, DTM2016, WTCR and free content i think are most popular, but you can find groups and leagues for pretty much any series