1) The "Matrix dodge" is only really applicable at long range when the target sees it coming. I had it happen last weekend at a game. My C7 rifle shoots 380-385 with .2, and about 320 with .28, which I always use outdoors. I took a long range (about 100 foot) shot at a guy with a burst of about 5-6 BBs. He saw them coming, exclaimed "OH SHIT!!!", and did the backwards Neo-dodge and the BBs scooted harmlessly past his head. But those are few and far between. Having .28s at that game got me more kills than I would have gotten with lighter BBs based on the wind and brush penetration alone. But that's a call you have to make for yourself.
2) If you get a decent gun off the bat, there won't really be a need to upgrade for some time. Start with a moderately powered AEG. Play with it and get to learn its strengths and weaknesses. You'll soon find out what you need to do to it in order to make it work better for you, and base your upgrades on that.
3) Plastic bodies are actually quite durable, and in many cases, better than low end pot metal receivers. As an added bonus, plastic doesn't contract as much in the cold as metal does, so you have fewer potential issues with playing outdoors in winter.