For the love of God, don't go SiG 552. Upgrades can be brutal for it, especially just equipping yourself with an ample amount of magazines.
Honestly, I do love my SiG 552. When I first got it, I was reluctant, but it was all that was available at the time. Over time, though, due to the immense amount of time I've put into it, I've fell in love with it. Problem, though, is that things such as magazines, are very hard to get. 5 of the realistic 43rd magazines with fake rounds in them will cost me about $250 and some change at my doorstep, opposed to buying, in your case, full-metal M4 magazines. Believe full-metal M4 magazines sell for like 90$ at your doorstep for 5? Correct me if I'm wrong, please, don't want to mislead the man, heh.
It's not just magazines that are difficult to obtain with a SiG 552. I find that the pistol grip is kind of flimsy compared to other weapon's pistol grips, so you have to be a little more careful with them. Everything else, so far, has survived what I've put it through. The only way you're going to get a top rail as well, is if you mount one yourself manually, or you dump the cash for a HurricanE 552, although worth the money, it is quite the expendature.
Motor's can also be a bit of a pain if you ever plan to upgrade one. As far as I know, the motor a SiG 552 takes only fits other SiG series guns, and the PSG-1. A lot of places I've visited don't even market a medium sized motor, so it usually requires an oversea's order, which I guess isn't too bad if you plan to do overseas orders. Tune up kits can also be a pain, as there isn't a big variety for the SiG. I've only found one on WGC, and it's not the type I'm even looking for (I'm going high-speed build). Result being you have to order the parts separately, which sometimes costs a little bit more.
Hope this helped for selection. I still h-- love my SiG 552, though.
Primary: Custom WE M16A3
Secondary: KWA MP9
Side: Custom WE M1911A1