I would suggest just getting an Enhanced JG M16 since they are cheaper, but they are out of stock. (unless of course you like the Stag Arms trades)
Air leaks between the cylinder and cylinder head can usually be fixed by teflon tape. I have a JG cylinder head that seals perfectly with an SRC type 0 cylinder once I added teflon tape.
Ehobbyasia.com is great for buying parts. (just for future reference)
E1s usually come with Madbull blue buckings, so there really isn't a reason to swap it out.
It depends on the model, but some E1s chrono at like 430, while others are in the 350 fps area. I would say 350 is probably the minimum for their armalite models.
I would imagine you could get close to 380 fps out of the stock spring if you fix the compression. (if the come with an M110)
I would say keep the stock spring (unless it shoots over 400 fps).
Systema and Guarder air nozzles seal with well with JG cylinder heads in my experience. Modify is similar to Guarder (they might even have the same OEM), but I haven't used one. If you want to replace the cylinder head too, (I don't really see a reason too) then make sure it is the same brand as the air nozzle. Mixing and matching brands for compression parts isn't always a good idea. (as tolerances are different)
Adding a sorbo pad is a good idea since it will help correct the AOE and can prevent the shell from cracking.
Spring guide should be fine. When you get it, check to see if the shaft of the spring guide can be unscrewed. If it can, I suggest adding super glue to the threads, then screw the shaft back on. The shaft on my guarder spring guide almost unscrewed all the way one time. Had I not caught it in time, it could have caused a lot of damage.
I used to be an SCS/Systema bucking fanboy. However after some experimenting, I found the Madbull X/Fishbone nubs to be better.
Because of the SCS design and material, it isn't very forgiving, so you get a lot of vertical deviation (because it's made of a hard material) and if it isn't sitting perfectly level, it screws up accuracy big time.
The SCS would probably work a lot better if the mound inside the bucking was removed. The SCS is good, but it's a bit too finicky in my experience. It can be difficult to get it to work right and when it does, it's probably no better than a KWA 2G, PDI W hold bucking, or another "fin" type bucking.