I think that it should be unlimited, just like in the tabletop.
But also like the tabletop
,customization should be a difficult, expensive task, and the more intense the customization job is, the greater the chance of messing up and horribly gimping your 'mech
For example, a swap of your Medium Laser for an ER Medium laser or a heat sink should be an easy customization, but still have a high chance of failure on the field (~50%), and a nominal chance of failure in factory conditions (~20%)
On the other hand a big job like swapping an engine or putting a whole new loadout should be a high risk even inside of a factory (~70%), and nearly impossible in the field (~95%).
Also, if your 'mech breaks - something bad happens... really bad. Stuff like your ammo might explode in it's bins if you get hit too hard, your engine might dump excess heat into your 'mech at a constant rate, your tarcomp might short out, not even leaving you iron sights to work with. Hell, you might even be ejected mid-fight the minute you take damage. Any of these sorts of things could occur, if it doesn't downright ruin your 'mech and make it useless for anything but a few unaffected spare parts.
And on the money side, I'd say that it should also be expensive. Field refits wouldn't be that expensive, but taking it into the factory and getting a major overhaul should cost triple the cost of the 'mech you're customizing
With that, you get the customization system, but like in Battletech, it's VERY rare, and only done if you have money to throw at it and are willing to lose your precious baby. With this system in place, we'd probably see some field refits here and there, but a fully customized 'mech would be a very rare thing to see on the field.
Something that would make an alternative to customization that would lower the risk significantly could be Refit Kits, produced by factories, that come with installation gear and the components required. These could be used to refit your 'mech into one of it's canon Variants, such as converting the Catapult with LRMs into the Kuritan variant that puts PPCs on instead, or to turn your Hunchback's AC20 into a Missile array, or even a battery of Medium Lasers.
Edited by ice trey, 23 January 2012 - 06:10 PM.