Alright, lets get started.
One, don't get the sling, thats for an M4a1, it won't work with the sniper.
Two, get a full face mask so you can protect your face--clearly. It will benefit you in the long run.
Three, although normally I would reccomend against getting "medium/low grade" snipers, I guess it's fine if you're just going to be in your backyard, but I have hard that the MK96 is not that good, I reccomend getting the one in the second link.
Four, Vforce makes a good facemask.
Five, don't get .3 BBs, it won't shoot far at all because its not a strong sniper, its a "beginners" one as sort of stated above. Real airsoft snipers will cost you a lot of money. Use .25s, if you must, use .28s at the max.
Six, I would reccomend against the scope, but if you're on a budget go for it. It doesn't seem to be that great but with your setup I guess it would work. It seems kind of mysterious and I personally wouldn't buy it, and would go for something more expensive, but have a name brand.
Seven, Personally, I wouldn't buy anything from China, which includes the gun and the scope, but it seems you're on a budget so that would hopefully suit your needs. If you have other questions, or I missed something, tell me.