I had one of these back in the day, in fact it may still be floating about the house somewhere.. have also owned a VSR-10 pro sniper and G-spec and can tell you there is not much in it. but yea as you say most everything for the Marui VSR will fit the USR internally aswell as externally, this makes it a very good, cheap alternative if you're just looking to dip your toe in the sniping pool.
I started with the Marui mount base, you can pick one up for about £10-12 depending on where you go. now this weapon doesn't come with sling loops (from what i can remember) and the little nipple thingies on the gun are made of plastic so they're not going to last too long. i filed mine off after they broke and didn't really miss the fact that they weren't there.
now the way i see it is you can go one of two ways
1. A smallish scope, a 4x fixed or something like 3-10x 30 or 40 will do the trick, nothing with huge magnification because it wont really make much difference (if you can see them you will most likely hit them) you can pick them up pretty cheap off ebay or from an airsoft retailer, some even come with mount rings.
2. A "Aimpoint" replica red dot. no messing, just slap it on with a nice low mount and away you go, no need to zero just point and shoot.
Option 1 requires a bit more work than option 2, you need to make sure the scope is mounted properly, you need to set the correct magnification then you have to steady the rifle in a vice of some kind to zero it to a range you feel comfortable and even then on a windy day you wont be hitting the crosshair.
Option 2 is quick and easy. and it also means you can be up close and personal black hawk down style :lol: but thats all up to you.
hope my ranting helps.
L1ferpr