Can’t comment on the IS, although what does that mean? Torsion axles instead of leafs? I see the advantage for longhaulers, lots of miles. Otherwise it’s a “meh” upgrade for the occasional user.
Brakes, if you had a good diesel with a good exhaust brake I’d say the brakes would be a nice addition but wholly unnecessary and more complicated (obviously more complicated which = more $).
Given you’re planning on pulling that unit through the biggest mountains on the continent with a gasser, makes having trailer brakes that are overachievers somewhere between a very nice luxury and possibly absolutely necessary.