Power-wise the 23 hp should be plenty for a 52". If you plan on bagging, well it should be enough as well... 25 hp wouldn't hurt though. If you are mulching only you probably better go with the 27 hp.
I doubt the fuel economy between these 3 engines is much different, ounces per day between top and bottom I'd guess. EFI is probably not an option on that model, is it?