you dont need an sr swap to drift, all you need is coilovers a diff and a seat, in that order perferably. if you really want to get into drifting, dont short yourself by buying a mustang that is not a good drift platform to start off with ( fox bodys can be made to handle but things you take for granted on an s chassis, like a good amount of steering angle for example, simply arent present on a stock fox body) an rx7, also stated before is comparitivly unreliable, especially a 1985.
cliffs: if you are serious about this, pick up a 240 and like fairgentelmen z said, go to a drift event