It's never good to leave a spring compressed for a long time. I also doubt they are using a real high quality spring to start with. Continually leaving it wound may lesson the overall pressure over time, but no short term side effects. You should be good, just shoot it and let it cycle every once in a while.
Fot the kit, you are looking for the G&P Pulse Rifle kit. Very hard to find in stock. Check with the popular airsoft dealers, UNCompany, Dentrinity, airsoftwar4, Redwolf, etc.
Coming from real firearms, there is no negative effect from leaving a spring compressed over a long period of time. It is actually use, compression and extension that causes a spring to wear out. Just look at magazine springs, recoil springs, heck, even the springs to a retractable pen can all be compressed for years at a time.
Maybe if it was made from garbage metal or experienced some weird heating issue or excessive build up of grime. But not in normal display or use environment.