1) Modification rules are mostly outlined here...
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First thing would be to calculate the Modification cap.
A 20 ton Mercury has a base modification cap of 2 tons (10% of it's weight), which is enough to remove the two lasers. If you wanted to change more you need to have something on your character sheet to indicate the character is bright enough to manage what they want to have done.
Removal of the (2) ML is pretty easy, you pay the labor to have them removed.
40,000 cb (cost of ML) x .2 (Mech Weight / 100) = 8k cb to remove one laser, 16k to remove both.
You now have two Medium Lasers lying around to throw in your inventory or to sell off. They're a common and popular weapon so selling it to a player or your unit inventory shouldnt be an impossible option. You're saving someone one an availabilty roll if you sell to another player, though selling to the market if youre lazy works too. If you have no plans to use them, either way you should make about 80k cb on the sale.
You need to have (4) small lasers on hand to install. Since this is a new character, making availability rolls for your starting mech is ignored unless youre looking for 'advance' tech (non-3025). Cost for (4) small lasers is 11,250 (x4) = 45k cb
Labour to install the Laser is same as to remove it.
45,000 cb (cost of 4 SL) x .2 (Mech Weight/100) = 9k cb to install four lasers.
Now you need to look at where they go. Removing the Medium Lasers gave you a critical location in the LT and RA. So your first two go into these locations.
You have two SL left to install and no open criticals. At this point all excess equipment needs to be installed on the arms. So you decides to put all the lasers in the RA, so you can keep the LA free to punch or put one in the LA, so that if you lose the RA you dont lose all the lasers.
17k for labor
45k for (4) SL
62k Total Cost, not bad when you consider you made 60k off the sale of the Medium Lasers they replace.
This is originally listed as a 2 ton modification, but Medium Lasers and Small lasers are both considered Energy Weapons, so you may count the replacement of a Medium Laser with a Small laser as a downgrade, which means that swapping out two ML for two SL counts only as One ton towards your modification cap.
2) Read message 6 ~
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3) MASC is considered Advance tech and you need to make a roll to find it. If you find it, you pay the cost to buy it, then if you have the space on the mech, you pay the cost to install it.
It should be noted that MASC requires a tech skill of '2' to use properly, for each point you lack, increase the chance for a 'freeze' result by one.
I'm making a logic call here, that installation criticals for MASC needs to be made in the legs, as long as you have an open critical to use in the leg, you may/must install MASC in that critical. If you do not have an open critical in the location you need to move something (1/2 ton mod) to make room for the MASC.
Look over the this thread for campaign interpretations of equipment, cost, and expected availability rolls.
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Engine modification, especially XL, are not to be taken lightly. I think XL engines are a fast track to the poor house, but i allow them. MASC is just tech. There is no available roll listed because the cost of the equipment varies which make the availability roll varies, i'm not going to work out the TN unless it's for a live project (this is all hypothetical). Having MASC free can also get you dead fast, not bad in a table top game where the mech is expendable, in a campaign game... it could mean starting over with a new character, if you accept these risk, more power to you but i dont want to hear it when your mech freezes in front of a Demolisher backed by a SRM Carrier.
