Here is my experience with Bilstein shock. Road wise the 5100 and 6112 offers very similar performance. The difference between the two show up when breaking hard and turning. With the 6112 you will get a little less nose dipping and leaning when turning at high speeds but the trade off is a little more stiffer ride. My 6112 give me a sports car like handling. I can handle high speed turn with ease and a much more cushier ride when going over bumps.
Off-Road you’re going to get better performance from the 6112. This is because the piston size is almost double the size of of the 5100. It’s designed to take much more off-road abuse.
For towing you’re probably better off with the 6112. This is because the bigger size of the 6112 can better handle the weight. If you want a more economical option you can do 5100 in the front and 6112 in the back.
Bilstein also has a new 8,000 series shocks. They are much beefier than the 6112 but I don’t really know much about them other than they are much more expensive.
For the coil springs I don’t think you will need to change them unless you add permanent weight to your bumpers. I added a 140 lb steel bumper to the front of my 4Runner and my front end sagged a little. The front OEM coils have a 650lb rating. I essentially upgraded it to a 700 lb rating with B-12 which is designed to handle a additional 80-140 lb of weight.
If you really want to use Bilstein shocks just call their customer service. Tell them what you want to do and they will guide you to the right product.
I found this link that provides good info on the difference between the 5100 and 6112
5th Gen 4Runner Shootout - Bilstein 5100 vs 6112 Shocks