Here, this is how mech lab should be done.
Weapons cost money, you can only add weapons in the repair bay.
It costs money to "dismount" a weapon, and money to "add" a weapon.
NOTE< dismounting and adding weapons should be done in design time, with changes being committed after the player knows how much it will cost (IE selling your ER large lasers only to find you dont have the slots to mount another weapon would sucks).
But make this process expensive, as expensive as buying a light mech, say have it cost 20 k to change 2 er meds out for an er large laser.
If you do it this way then only the people who are doing well and purchased up will get into the arena of being able to customize their rides, but will also allow minor tweaks for mechs people want to have more specialization.
IE, take a mech like the Uller with the gaus.
You could go out, fight a little, come back and only have 20k and replace the er meds with 1 er large, or remove the ER meds and add ammo capacity.
Something like that.
If you limit the lab part of the battlemech, and make it something that can be done fast when at the repair bay (so people dont spend hours tweaking their mechs instead of fighting) then it would be good.
