If you're looking for cheap, from what I recall 12V drills will be the way to go - the cheap brand ones are mostly 12V-ers I think. I'm sure other people will be able to offer advice too, but I'd say to go with whatever voltage your weapon system uses if possible, makes things a lot simpler if you have a single battery.

Second question, what do you mean by 'best'? It depends on your skill level, if you're not fully clued up on pneumatics I'd not jump straight in with those. Not entirely sure about springs still, but would be an interesting one if it worked well scaled up... A lifter wouldn't be too difficult and would work nicely for a first machine, if you wanted to go the linear actuator route. Weapons can be interchangeable too, I suppose you could make several lifting arms or what have you and swap those out as needed, one with spikes and then another with a bulldozer scoop, then maybe one with just a large spike - that sort of thing

Featherweights come in all sizes too:


These are both featherweights but Tormenta 2 looks absolutely giant compared to Hardwired 2. What size is your model so far? I'm not sure what a 'typical' lifter/flipper type robot is (they're usually a tad bigger than others) but Hardwired 2 is about 300x400x50(80 if you include wheels) from what I remember so as long as it's bigger than that I'd imagine you're on the right track haha.