King Arms M4s are solid. The only stand-out failure point I've seen is in the wiring harness, specifically the contacts, and only KA trigger-blocks/ harnesses like to play nicely, but that's after an abnormally high amount of use.
A KA M4 should run just under or above $300 all-said, but please don't order from the 'States or attempt to smuggle one. Search around the forums (the threads are stickied) to find out why. The risk/ reward factors are very heavily favoured towards risk in all cases, and the risks are life-long in almost all cases.
Lipos are awesome, but will kill just about anything real quick (relatively) without modification, and can be dangerous. Stick to 9.6v Nimhs to begin with and do a bit of research on Lipos before you want to commit.
Age verification takes a while, but it gives you more time to save and think, and opens the doors to great retailers in Canada. If you can play at a place where you have the option to rent, do it! Try different systems. You might find that you like the ergonomics of a different platform more than the M4, and you can save a lot of time and money by trying before you buy to find the right system for yourself.
Gas rifles re super-duper sweet, but require more nitpicking maintenance and getting enough mags together can get stupid expensive reeeeeal quick.
Hope that helps!