if you could, go with a sump. You can DIY one and it gives you the best bang for the buck.
other option is canister filter, personally I have had great experience with Fluval FX5, the other canister filter brand choice would be Ehiem. Both are great filters.
as for how many rbp in a 125g, personally I would start with 12 baby rbps. you can down size if they all live to adult hood and you think there are too many of them in the tank. Some like to over stock the tank a bit, some like to give maximum room for each individual fish so it really depends. they do feel more comfortable in number so as long as you can keep water parameters in check you should be ok, IMO.