FPS doesn't actually mean range. It just means that the BB will reach the target faster. There's one member actually who has a gun running stock FPS (330 I think) and outranging other guns. Just remember you'll need to lead your shots because you're shooting slower.
As for the sound, you can do a few things to mitigate it but there will still be some sound. Proper shimming, possibly switching to helical gears, and setting your motor height properly are a few things that will quiet down your gun. A suppressor likely won't do much except for the sound of the BB exiting the barrel because most of the noise actually comes from the gearbox area.
I don't know about upgrading the spring but it's best to not IMO. Sure it's an instant upgrade to FPS but you have to make sure the gearbox can handle it and not break within the next few hundred shots (more wear, shock, and strain to the gearbox = shit breaking faster). You'll probably have to upgrade any plastic internals to metal ones and good quality metal ones not shit monkey metal.
You don't have to change the cylinder but it's recommended to get a cylinder that matches your barrel length.
With the ICS you've got the split gearbox design so you can pretty much just have 2 uppers and switch between them in like 5 minutes easy so if you play a lot of indoor and outdoor it's probably good cause you just pop in your upper and you've got the right length and speeds for CQB.
With the SRC though, it's been reviewed by a few people and they say that the 3rd gen SRC's are great, better than anything else currently available publicly without being AV'ed.
I really push AV if you're willing to pay a little more for much better quality (take a look at the FAQ's and the top of the newbie tank for a general "price" table for what to expect in the classifieds).
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE