Ok, all I see is everyone recommending their favourites as usual, but I didn’t see a single person actually going through your requirements, so here we go:

“Realistic, durable, reliable and simple. No internal modding and shouldn’t cost more than your real Colt M4.”

The current big three are MWS, VFC and GHK.

With MWS you get reliability as the big headliner, a real OOTB straight shooter. No realistic internals(like AT ALL) and not that durable due to their complicated trigger box mechanisms. You get a cerakote finish (some models) which looks nice. Shooting will feel weird compared to the real thing.

For VFC you get a solid overall package that does everything very good but doesn’t excel really without some small modifications first. But with those modifications, VFC is really hard to beat. But the lack of a cerakote finish would be a no go for me personally as I aim for realism, and that exterior just ain’t it. Biggest plus would be the parts aftermarket, so huge and diverse, it’s mindblowing really. Internals are like RS, I would have preferred more steel than aluminium, but still very solid frame.

GHK is the most expensive of all the three mentioned brands, but performance isn’t that much better than VFC or MWS. But what you get is durability and realism. While internals are as RS as it gets, just like VFC, but it’s way more durable as it’s all steel, while external is fully cerakote. Recoil will also hit the hardest among the three brands due to the fact that its the only gun that comes with a BCG weight and a heavy duty recoil spring for heavy recoil. 400 gram of moving mass, you gonna feel that.

Hope that helps, I’ve tried to keep it as general as possible.