I wouldn't get too hung up about the cost of brass and availability (within reason). Once you have 100 cases, it is going to take you a lot of shooting before you wear the brass out with cast boolits.
My choice, 30/30 or 308, but anything 30 cal or over will do the trick. The 303 and 8mm are very cast boolit friendly, but there are more moulds available in 30cal.
Your ultimate choice will no doubt be dictated by what rifle you end up with and /or can afford.
If you are financially challenged, the Mosin Nagents make a cheap, boolit friendly choice,as do a lot of bubba'd milsup rifles. They ain't pretty, but if they have good bores, and are cheap and you are squatting in fine cotton as one of the guys here says.