I have done two different setups with regards to electric fans on my Mustang, both using small dual fans. I first did a pusher setup (fans infront of the radiator), and it worked ok, but it looked better than it performed (should I chrome it?
). Those same fans were too deep to put on the other side of the radiator, so I put in two other thinner fans on the engine side (puller setup) and took the pushers out. Works much better now, cools the engine off no problem now.
The fans that are currently in there came from a Ford Escort I believe, they are 12 inch in diameter, and have their own metal cage/frame built around them. The fans were for some auxilary thing in the cars, not all Escorts have them, so they take some hunting to find. And depending on what kind of radiator you have, they may take some fudging to get them to fit. You will definately need to make some sort of bracketry to mount them up, but keep it simple and it will be just fine.
Also, pick up an adjustable temp sensor switch from your local autoparts store to control them (or mail order), and make sure you use a relay that actives the fans only when the key is on, otherwise the darn things will always be running after you park your car.
You may be able to get away with using one big fan, but I found it much easier to find two small fans and adapting them to fit wasn't all that difficult. Oh yeah, and as far as price goes, I think I paid about $15 per fan. Hopefully some of my ramblings will help :p