Hi Dan,
Where to begin?
What is your current level of skill in Driving sims? What have you driven before? What kind of hardware have you got? (Wheel?, Controller? Keyboard and mouse?)
With regard to URD content, it is worth every penny you pay for it. It currently is the only payware I am aware of. I would start with their EGT content. The T5 is no longer being developed but the EGT and PX content is still under development and gets periodic updates.
We use the APEX mod content. It is very good for GT3 content and we probably will stick with it since not everyone has EGT and the Simtek Mod has some questionable origins. But if you dont have any personal hangups with possibly swiped content then they are a fine group of cars as well.
Take your pick of tracks. Any of the tracks we have used in this series are available from my dropbox account. Each track is linked in the races information page.
I am sure a better list can be posted by one of our seasoned drivers and posters.
In the end I would suggest trying all of the content you can come across. Yoiu will find the mods and tracks you enjoy right off the bat. Once you have discovered the mods you enjoy you can develop your driving style based on the cars you like (BTCC, GT3, F!, Stock Car, Etc.)
Good luck with your sim racing career. Ask any more questions you can think of. We are here to assist. Also feel free to join our weekly race server (VRC GT3 Series) to post some laps and times. See how you measure up to experienced and not so experienced drivers.
~Chad