Stiffness of the springs very much depends on the engine and specifics of the car. OSS adds something like 35kg, and much of that is towards the rear of the car, so the weight requirement goes up. TDIs weigh more over the front axle, so need stronger springs there.
Some Spidans can push the front of the car up, and availability has been seen to be spotty, with others expecting an even softer ride than before;
for reasons explained previously, I don't personally think that's a good idea and the options are actually limited anyway.
B4s are a good compromise with apparently decent longevity and a ride that is similar to stock. B3s are considered to be the natural partner to B4s.
If you want better ("sportier") damping, then B6 is the way to go; B8s are similar but with a shorter piston and so set up for a significant drop.
The ARBs are great. No sideways roll any more, gravity pushes you sideways. I would consider changing anything and everything at close to the same time: make sure you mark the position of the aggregatenträger / front subframe when you drop it to put the ARB in and make sure you use the correct stretch bolts when refitting. I run B8 + Eibach + ARB + Superpro + light 15" and 195/50. It's a go-kart.
I would strongly recommend an alignment afterwards; the subframe will limit the amount of movement you have and there's lots of information about this over on the German forum, including ways to get a PQ25 front axle to fit, but it's convoluted and difficult and you'd need a pretty dedicated garage to want to do it for you.
For the front top mount bearings, I would not go Bilstein but stay VAG with 6Q0 412 331B . That's a HD mount and should last well. I would also replace the droplinks at the same time with Meyle HD ones.
For the ARBs you can also use the Superpro bushes, they work very well indeed. I also happen to have a set of rear axle bushes from them for fitting when I'm feeling bored and want to take the rear axle off - I haven't got around to it but I'm reliably informed they also help.
Those are my thoughts, anyway
- Bret