That eBay guy is certainly Mr Fan (good prices too, I think).
What will work depends on how you plan to use it. Is it in addition to the stock fan or in place of the original? As a pusher or puller? Thermoswitch controlled or manually controlled?
Size will depend on the radiator... TR4s can use a 12", apparently TR6s use a 14", don't recall the width of a 4A rad.
I drive all summer. I get overheated, the car does fine.