scope can just be whacked onto the rail, provided you have scope rings. Don't get anything that goes too high though, its unncesary. aim for something around 3X magnification.
as far as telescopic scopes go, they have a number such as '3-9X32' or '12X50' something like that. The second number (in these cases '32' and '50' are the diameter in mm of the front (objective) lense. the bigger the better for this as a larger diameter will provide a wider area of light to enter the scope, thus making target requisition faster and easier. the first number '12' for example is the magnification power. DO NOT EXCEED 4!!!! but that is only on a fixed magnification. On a variable '3-9' (quite a common size) you can magnify from 3X to 9X and anything in between via a spining ring. As long as you can get below 4X zoom you should be fine and the larger the first number the better.
incidently, whilst many people are like 'sweet, you have a REAL dragunov scope' I am not. they cost too much and are, for lack of a better word, utterly s***. they are around an 8X zoom on a 24mm objective lense, meaning you can see the bloke you are going to shoot, but you can't find him with any ease at all. avoid at all costs.
Personally on an AEG I would just stick a red dot. I happen to own a 1X50 red dot scope. it works fantastic. failing an enclosed dot, try looking at a reflex scope or a holographic scope, just remember a good rule of thumb is
never put more money on a scope than the gun so avoid holo's.
If you want a telescopic scope (an actual scope) find a local gun/hunting/knife store and have a look at some. Don't get swayed by illuminated reticules, they may look pretty, but batteries for them are mega-expensive and they are fairly useless in a game. just more weight. also, the simpler the better, you don't need a million differnt knobs and dials on the scope, its a waste of time, effort and money.
most scopes off ebay will work fine for airsofting purposes.
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anyway, enough on scopes.
as far as other upgrades go, new hop up unit may be in order, I know my mates one is crap and BB's are just EVERYWHERE. Don't know what type it takes, although I would hazard a guess at a marui AUG.
upgrade hop up before barrel.
mags - almost anything that fits a marui or a copy of a marui will fit. may be worth calling/email the shop before you place an order.
cheap but quality attatchments - China/HongKong. Just make sure any lasers do not exceed 4mw
Well that was a fun 20 minutes