To add a little to what DanB has stated...
As for the cooler - as with most all late model cars, almost all of them use a 195 degree thremostat. With the engine running at that temp and without regular system flushes or fluid changes, you can only expect your tranny to last around 50K! And that is in everyday driving. Drive you car hard and you will cut that almost in half.
An aftermarket cooler is a good thing. Even a small one, such as the smaller B&M Super Cool cooler that you can get from Summit for around $50. Anything outside of the rad is a good thing!
As for the line filters - If you do a search here using the work Magnafine, you will find tons of threads on the topic of this filter. A few have them installed and as far as I know, they are not seeing any problems as of late with the filter causeing problems with the tranny. Many do agree though that the spin on filter design is much better since it has less of an effect on line pressure.
As for what is exceptable line pressure varations - From what I can tell in the Ford service manual, you need to have between 45-63 psi at idle in drive. That would give you something like 18 psi to play with.
One problem with the spin on filter is that unless you move your battery to the trunk, you really don't have much room for it anywhere else other than where the battery currently is.
I personally don't have either of the filters. I just change my fluid out like a mad man!!
Welcome aboard and hope that we can help you are your friend with anything else.
