Hop up should be set to something NOT to none or lowest.
What kind of accuracy are you expecting? This is airsoft and not like a pellet gun at all, you should expect the same performance as a musket rifle in terms of accuracy as it's the closest analogy you can get because they're both smoothbore guns that fire spherical objects.
Go with modify parts if on a budget, http://www.modify.com.tw/
cheap price but good performance for what you pay.
BUT first get a good hop up and nub then a new barrel and you'll see a big difference. Then go with compression unit (piston, cylinder, piston head, etc.) to make it more consistent.
Also do not upgrade the spring, stock most clone guns fire at a pretty good level for outdoor play (~370-390 FPS on .20's) and remember it's a limit not a goal.