These expansion packs basically add 2 car (or bikes) and 5 trophies, right? It says there are some more events, but I believe they are just the usual events but with these new vehicles? Or is there anything special on them? If not, I will be skipping, DC+Season Pass + Bikes is already good enough for me.
Each tour pack adds a certain number of tour events, cars and trophies. These last few that aren't included in the season pass added 6 events, 2 vehicles and 5 trophies each, while the ones included in the season pass and most of the free tour packs (there are a couple of exceptions among the latter) added 11 tour events, 4-5 cars and 5 trophies each.
In short, you'll be set for a very long time with the base game, season pass and free content and if you really enjoy what's on offer then you might as well get the remaining tour packs and DC Bikes when they go on sale.
Just a warning about the tours. The tour mode in the base game and the first 4 (i think) dlc expansions don't have weather effects like rain and snow, since that has been patched in after launch, so if you want to experience those, you'll have to play the season pass tours (or set up your own exhibition races which let you customize everything), starting with the Elements Tour. However, the difficulty in most of the season pass tours can prove to be a bit challenging for newcomers, so you're better off going through the Drivceclub tour first, which starts you off with the slowest class and has a more gradual difficulty curve.
Also, there's a tour called Startline, which was meant as a sampler for people who hadn't bought the full game yet. It features a variety of tour events from the main Driveclub Tour, as well as the season pass. However, don't let the name fool you, the events in the Startline Tour are also a bit of a mixed bag difficulty wise, so don't actually start off with that.
By the way, if you want to try out how the bikes feel before buying the expansion, there are 2 free bikes avaliable to everyone, even if you don't have DC Bikes. The Aprilia, whiich iirc is avaliable from the get go, and a KTM Bike, which can be unlocked by driving other KTM vehicles.
That's all for now, i think. Come back and let us know how it's going for you, and don't get frustrated, it might take some time to get a handle of the driving model if you're not a racing game buff, i know it did for me. I'm sure you'll get some useful advice here.