Regarding must-haves for upgrades:
The original kit we said servo horn and wheel hexes. This kit appears to already have the wheel hexes.
For springs and jumping:
I always liked a stiff setup on the 410, but with the shocks further out on the arm and tower like they are with this kit, less spring and less oil weight is needed to get a similar damping effect, which is why you see them set to lower than with the original kit. While I wouldn't consider it a must have and I will run the stock setup before trying anything else, I would probably have 1 spring up/down, some different shock oils, and maybe a different piston to have on hand and try.
Regarding composites:
Most people like upgrading to steel or aluminum, but this adds weight along with durability. That being said, many other manufacturers also use composite cross pins (they're not really taking much in the way of force), and some even use composite gears. While time will tell if these hold up, I don't see a specific reason why they shouldn't.
That being said, I'll be building mine with metal cross pins in the diffs just because I like to "set it and forget it" (for up to a year long period of time) and still have the diffs look brand new inside when I pop it open to replace an outdrive. If you do semi-regular diff maintenance, I don't think the composite crosspin will be an issue.
Regarding Price:
Tower has always had pre-order or first displayed pricing incorrect, so I wouldn't even bother looking at the price yet. Expect it to be about the same as the prior kit was.