Realistic, short and lightweight in the same sentence can't coexist.
I went with Specna Cores for my M4s because of the polymer body and QC 8mm gearboxes, also they are silly cheap (paid €120 for the one pictured, 150 for my other M4), over time I have replaced EVERYTHING inside them, including gearbox shells.
You can pick up pretty much whatever you want, G&Gs used to be great bases to work with but as of today, there are plenty of alternatives that give more bang for buck (Specna, Dboys, Evolution, Saigo - these all come from the same factory, same guns different coloured plastics.. Cyma is great too).
Upgrade wise, find out what you want from your gun first and then upgrade as needed.
It doesn't make sense to empty the whole thing and rebuild it if you don't know what you're looking for, you will just waste money and have something that doesn't put an evil grin on your face every time you plug the battery in.
My M4s, for example, are twin builds as I've put the same exact internals in both, bar the inner barrel and spring. The train of thought behind them was to have the most reliable gun possible that could still provide a decent full auto ROF of about 20 to 24 RPS, decently snappy semi auto response and be as battery efficient as possible.
I've gone for Warhead 36k,Retro Arms gearbox, SHS/Rocket 13:1 gears, 8mm steel solid bushings, FPS compression parts, Perun Hybrid FCU, Maxx ME hop units, Maple Leaf 70° hop rubbers.
As mentioned above the only differences are found in the spring rating (M90 for the long boi, M100 for the short) and inner barrel (6.05x247 and 6.05x185 respectively).
Externally I tried to keep things light, so the choice of stock was the King Arms PDW (which is now impossible to find anywhere but from KA themselves and good luck buying one with their insane shipping rate), carbon fibre outer barrels and AliExpress handguards (back when you could find them).
The carbon fibre stuff I source from Mac Airsoft, which I happen to live just 40 minutes away from.. We're both based in Italy but he does make CF barrels, stocks and handguards for many brands (Monk Customs and the likes) so you will find his stuff pretty much everywhere. He also makes very neat CNC'd aluminium receivers, but I prefer plastic.
My guns aren't realistic at all, they're just functional to what I do with them.
Pic of my other M4 for boasting purposes (2.2kg)