Hey mate,
Can't help you in specific Megane 3RS questions but in general, reliability is pretty good compared to other Euro marques. It may not be Honda levels of reliability.
The biggest difference to the K20a motors you will find is the timing belt. Renault rates these good for 4-5 years depending on the motor. This is where the need for a specialist is usually required. As the cams require a locking tool. Thankfully there are quite a few specialists around, especially on the east coast.
The other observation is engine mounts and suspension rubbers (top mounts, lower control arms, tierods etc) tend to require replacement more frequently. Once again this is my observation from owning a Clio 172 and 200.
I'm sure somebody will be along shortly to answer your Megane 265 questions specifically.