Everything I've seen on aftermarket fans says that SPAL is at the top of the heap for quality and performance, especially if you spring for their programmable controller and temp sensor as a total package. Go to their website to get started, but then you can call their tech line to get one-on-one guidance for selection, and for any operating questions that might arise. A top notch company, in my book.
Like Gary says, a lot of folks have done well using OEM fans from Fords - seen lots of Taurus fans with using OEM relays for a low buck approach that seems to work, and there are lots of them out there.