Prometheus 110 spring period.
I'll tell you something. PDI springs are very inconsistant (do you want 350fps or 425fps? Get a PDI 150%). I installed a GOOD (350fps on the nose) 150% spring in my first MP5, within two months of shooting in the basement (was over the winter) my stock mechbox broke bad. So I bought a Hurricane reinforced mechbox (very good mechboxes!) and it lasted me about two years worth of playing, then busted (remember, reinforced mechboxes just take longer to break) that one too. Now my MP5SD is in pieces waiting to order a new box. Mind you, I was getting 352fps for the first while, put in a bearing spring guide knocked me up to 365fps, and I put in a 330mm inner barrel took me to 387fps last time I chronied it high.
I got a well beaten former rental MP5A5 last year, stock mechbox, and I upgraded it with only metal bushings and a Promtheus 110SP spring, I've been using it all year (that's what, 15odd games?) and it's still going strong, mechbox is intact still and still chronies at 365fps with 0.20g BBs.
Decide what fps you want out of it (if you say 400fps I'm gonna slap you
), then we can help you along the path of basic cheap upgrades. I have no idea what standing or parts are required for the UTG brand, but at least yo uhave a clearer picture of what to get and what to avoid.