As long as you made sure it worked before you committed to the purchase you wont be totally boned. Its not like the gun is total absolute crap. Not the greatest quality but well definitely not the worst out there. In fact I believe it may even be a rebranded Jing Gong brand gun. JG brand guns are not terrible, on my personal quality scale they sit somewhere at or just below the average line. If it was well taken care of by the previous owner all you probably need to do is give it a once over basic maintenance.
The sig 552 uses a standard V3 mechbox. Its nothing special. It should be a clone of the Tokyo Marui made model so parts should be interchangeable. So you should be able to use TM magazines as well.
Your request for the users that hate the gun not to post is strange. If someone has owned used and hated this item it would be a good idea for them to share their experience with you instead of leaving you in the dark. I can understand if your comment was direct at those who just hate on the gun without ever having first hand experience though, but dont worry this section of the forum is for constructive answers only. No flaming or asshattery is allowed.
"Its only a little bit on fire"