1. while you are at it replace the OEM fan clutch with the Hayden 2905 heavy duty unit (cuts in earlier than OEM is engine starts to heat up) and mod your fan shroud to make future access easier see
viewtopic.php?f=98&t=64278&p=709322#p709322 for suggestions and links.
2. see
http://www.dieselglowplug.com/gx3123-di ... svli5qq877 for drop in 7v replacements (no ECU reflash needed) cost ca. $180 US delivered.
3. wouldn't hurt if you want to tackle it. I don't have a write up but it's a bit involved IIRC
4. what company?
5. if it's just hwy no heavy towing go with the EcoTune and standard TCM tune. Most of the benefit in engine life and performance will be from the EcoTune. The TCM tune helps a bit mainly by restoring something like the pre-F-37 shift points and a bit better mpg in town, at cruise on the hwy (say 1800 rpm) it doesn't add anything