Be warned that Shimano sandals are actually quite warm due to the thick insulating soles (which are stiff enough for decent pedalling). Well ventilated roadie shoes can actually be cooler.
In either case, carry a spare pair of socks for (after) the rain.
SJSC also sell (or have sold) various waterproof overshoes. I have some Carradice ones, I think they are, made of real ducks or something. Strange flappy shape, though, that needed a bit of alteration.