I had a G&G M4 pass through my hands on the way to a friend last month with a plastic receiver. I was very impressed by the quality of the plastic.
Some companies plastic are far superior to others (Compare a JG G36 to a CA G36). G&G offers a very high grade plastic on their M4s.
However, if you are looking at keeping your gun in the long term (5 years as you indicated), it would be worthwhile to invest in a metal receiver. Metal is more durable, and as m102404 pointed out, upgrading to an aftermarket receiver is quite expensive. ($200 minimum)
Hope that helps!
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