I'd definitely be going down the secondhand route, tho with anything it's a gamble if somebody's been messing with it.
Personally I'd be looking for a s410, ultra, s200, s400, falcon raptor, super 10, artemis m16, even a few krals.
Also imo, pumping a cylinder is easier than a 500cc bottle, tho if you top up after every time or a few mags, you don't spend half the day like a jack in the box. Let the pump cool, don't go balls out and do it in a oner
Have a gander in the for sale bit and see what comes up
Super soaker 3000 (water), nerf fang (foam), noisy cricket (energy), m41a pulse rifle (10x24), Gat gun (.177)