Head work is more so a matter of how much do you want to spend!!! By the sounds of it you can got pretty hard on it because of the block its going on. Just the usual stuff like port and polish inlet ports, 4k etc exhaust ports are tiny so open them up a bit to, CC the head, bigger valves, 3+ angle valve grind etc etc. As for skimming it you really need to work out your CR and see weather its really required.
As for the gasket, whats the diametre of the 5k gasket vs the 4a pistons?
Engines don't really come out of balance, unless something like a harmonic balancer is playing silly buggers.
There's a couple of main areas of balancing motors. Firstly weight matching things like rods and piston, so there isn't say 1 piston that slightly heavier putting more load on one part of the crank. Then you've got balancing the rotating assemble. I plan on doing this when I finally get around to rebuilding another 5k I have. Going to balance everything from the gilmer drive pulley>crank>flywheel>pressure plate.
The thing you would want to watch out for is weather this engine has been balanced with a certain FW on it! Did you get one with the motor?