Being a relatively new Savage owner I can tell you what I did.
I broke the truck in right out of the box with no changes. After break in I ran it for a couple of tank before I got up the nerve to tear into (all I ever messed with in the past was on road)...
I pulled the front half of the truck down and installed the cam-servo saver...
A couple of tanks later I put the four gear diff upgrade kit in mine with 5,000 weight oil. I just bash in dirt and jumps but it gets really good traction compared to before. Maybe I should have only put 3,000 in it, but it was suggested to go to 5,000 and at nearly 10 bucks for a bottle of the Ofna I wasn't going to buy two!
I also installed the dual disc brake kit which gives it insane braking, you have to really watch it now.
I also did Mayhem's fuel tank protector tip
https://www.rcnitrotalk.com/2005/savage-fuel-tank-protector/
I haven't done the linkage mod yet, but I think I'm going to wait and do that when I upgrade the servos and once I see a modified truck in person.
I also plan to do the Uniflow mod to the tank
Other than that it is stock and I'm having a blast with it!
Oh, if it is a rotostart equipped model you'll need a 7.2v stick battery for that and you might want to pick up an extra glow plug or two (though mine is on the original still).
Good luck!