Flippers are pretty costly and very complicated. It's not really as simple as sticking a ram onto a hinged piece of metal, everything has to be thought out. Experience is a must have. That said, there's only one way to learn, right?

I think a good idea is to build a simple first robot, leaving enough space and weight to add a flipper later on, going for a pneumatic flipper robot first time is quite ambitious, and you might decide half way through the build you aren't happy or you no longer want a flipper, meaning you will have spent a lot more money than was necesarry.

Another thing is, a well angled scoop/ramp/wedge can flip other robots over if your robot has the speed and power to push them or hit them hard enough, or another option could be an electric linear actuator (I've heard they're pretty cheap, I might be very wrong though), they aren't very fast or powerful for that matter, but I think I also heard somewhere they can lift 30kgs? If that's the case (again, don't take my word for it) then a 13 or so kg robot shouldn't be a problem (taking into account the power lost in a hinge mechanism and the lengh of the lifting arm etc).

I'm just starting too tbh, all I just wrote might well be cobblers, so have someone more experienced than I tell you what's what, I'm just going by what my common sense is telling me, lol.