The D3200 will definitely hold its value for longer, being that much newer, and with the latest generation technology inside, but that's a good point about availability. You'd need to get it by the end of May at the latest I guess and I'm not sure if that's do-able. I'll make some enquiries.
As for the budget, I was working on your quoted £750 limit, and with the camera at £650 and the 50mm at £100 it does just fit, but if that's too much the kit lens is perfectly fine and it will do you until you can afford the 50mm.