You don't need a rail system for a G3.
There are several clamp on rails that go just in front of the rear sight. These come in a few lengths and styles. And don't have to be expensive.
The most basic type is about £10, And lets you use an optic. If your optic has a raiser block with a sight hole in the riser then you should be able to use the optic and iron sights.
http://www.patrolbase.co.uk/airsoft-bipod-mount/asg-aeg-mount-mp5-g3.htm#.WyhYSYrTVes
For the front of the gun you have far fewer options. The slim hand guard like in your picture has no upgrade capacity for extras. There are other variants and a couple do have RIS systems for the sides and lower but normally not the upper.
I would pick the variant you want from the start, rather than trying to get hold of handguards that have other options.
If you want something a bit more special have a chat to
@Jez_Armstrong
He does a fair bit to G3's including wood kits, I don't know if he does them for sale or just for himself, but if anyone knows the best of the budget guns it will be him. I believe he uses classic army guns and modifies those. CA make a few variants on the G3 from the standard A3 slim like the one in your picture, right through to short tactical variants.
http://www.classicarmy.com/product_sub.php?cid=266
If you want gun porn go have a look at the LCT G3 range. They recently had a price drop from £400 to £329. But the guns need a bit of a tweak to get them right, so I wouldn't advise one for someone that can't tech themselves. As out of the box the tappet plates need changing, and the cylinder head will need some glue putting on the washer. It'll also want a shim and probably a mosfet. If you can do all that then the LCT has the hands down best externals on the market atm.
https://www.fire-support.co.uk/product/lct-lc-3a3-s-g3a3-green-od-aeg-airsoft-gun
The other option if you want forward ris is to look at something like the classic army SAR. It's still a G3, but with a front that allows for a torch and vertical grip. It also has a metal upper and front end unlike the JG that is all plastic and known for been a touch creaky. The 230mm barrel length shouldn't put you off, you can grab a long suppressor 140mm+ and use it as a barrel extension to get the inner up to a decent length.
https://www.fire-support.co.uk/product/classic-army-sar-offizier-m41-fs-airsoft-gun-aeg-sale
