I use any mod that I have lying around (if it fits) as it will increase the block damage.
If you are min/maxing Ergo Grip and Iron Breaker (not sure it works for more iron while disassembling), and the others don't matter as long as they are filled.
By the time a player has a Tier 2 tool, repair kits are trivial, so the Structural brace is not required but will fill a slot.