You realize that you do need to go 100mph for the 100k celebration don't ya?
As for the tranny cooler
www.summitracing.com sells quite a few that should work for ya. I have a B&M 19,000gvw 3/4" cooler between my condens and rad. I see temps that will just keep going up above 200 degrees in stop and go traffic and some large long mountaions but cruised decently on the highway (140-170). In the winter it rarely gets up past normal op. temp which is about 140. Usually around 120-130. I'd recommend a gauge to so you know when your frying the ATF. I have a norsdouq???? digital gauge (green almost matches the color of the svx's lights) with the sender in my pan. Others recommend it elsewhere which is probably easier too.
As for plugs NGK all the way baby. 1. They are what Subaru recommends, 2. They are fairly cheap, 3. I've never had an issue with one in any of my Subarus. Bosch always seem t increase the boxer hesitation to me.
Do some searching on the sight you'll come up with a weath of info on both though questions though. Congrats on the milestone. Be sure to register it in the Subaru High Mileage Club!