The scope needs to have a bandwidth of at least 5 times the maximum frequency you want to look at. The bandwidth is the -3dB point. If the signals are square waves you need several harmonics within the bandwidth else it will no longer look very square.
I bought a 100MHz Hantek from Amazon for £220 which I am very pleased with. I doubt if anything cheaper than this will be much use for RF. I expect it's cheaper in the US.