I don't remember the assembly of the idler, but I do know I only threw a track once in 10 years and that was with a sizeable stick getting jammed in the running gear.

The idler adjusts by two screws in slots. Granted it's not their best design since the tension pulls down the slot, but with a little PVA on the threads and slot before tightening, it won't move (at least mine never did and you guys saw my son drive) until you take a screwdriver to it.

My point is, most people have a bit of sticker shock with their first Tamiya. Don't feel the need to pour a bunch extra into it unless you want to. If your idler becomes a problem it won't be right away (again, mine never did) and if it does, it's an easy switch out at anytime.

There's no reason not to install the elev arm upgrade off the bat since you have it, but I'd build and enjoy it for a while. The only irreversible thing you could do is glue in the bushings. If you want to use bearings, you should do it now.