Thanks for the response, that clears up a lot, and I didn't know about the 100-ton per bay capacity. I think I would go with a couple of Unions, but I might make that decision a diplomacy encounter all it's own between the PC's and the Dropship scheduler on Winter in the midst of the chaotic scramble of the early clan invasion. If things go well, maybe they get an Overlord and a Union; if they go bad...a pair of Leopards? :OP
I'm still curious about the weight of a tech and medical team; are they 1 ton each? And does food/etc., take up space in the cargo bays?
Each 150-ton Mech and ASF bay has to be able to hold a 100 ton Mech, so it needs the cargo capacity to do so (the remaining 50 tons is eaten up with general repair and maintenance equipment). If you carry a lighter Mech, that tonnage is freed up.
Medical teams can be simulated by using the gear from Tech manual, for the surgical theater plus beds. Actual medical personnel will need to be either in Bays, or in Quarters, and will need food and paychecks. Not sure about technical team mass.
Food for people in Quarters is 1 ton per 200 days. I don't know the tonnage of food/life-support per man-day for people in Bays though.
Your best bet might be a customized Mule Dropship with 12 Mech Bays, but a note saying only 4 can drop Mechs due to needing to make it look identical to a regular Mule. Similarly, it would have 6 ASF Bays, but only 2 of them can be launched at a time. Say it takes 5 minutes (AT2 turns) to move another ASF into position. Again, this is to hide the ASF launch bays from the outside. Maybe it is a case where ASF airlock 1 services ASF bays A-C, and ASFA 2 services bays D-F. You can't store a fighter in the airlock bay, so whichever ASF is in Bays B and E are the first out. When the ASF in A wants to launch, it has to be moved to the empty bay B, then launched (or spend half an hour physically swapping them if you want to launch the ASF Bay A fighter but not the ASF Bay B fighter).
Give the Mule an acceleration and armor upgrade (Thrust increase by 1 takes up 728 tons), but don't increase the Threshold (so antishipping ASF or enemy Dropships are still a threat). With the acceleration upgrade, 12 Mech Bays, and 6 ASF Bays, that takes up 3428 tons of the Mule's 8500 tons, leaving 5072 tons available. Not sure about the rules for adding armor,
sarna.net has it with 300 tons of standard armor, but my copy of Techmanual says the max for the SI 20 Mule would be 72 tons (and the armor listed would only take up 21 tons). You might be better off just taking the Techmanual and making your own, then giving it quirks/notes about why it isn't as good as your custom vessel. Don't change the number of weapons, but upgrading them to LB-5X, Streak SRM-6, plus similar upgrades for the lasers, with extra ammo for the AC and SRM would be nice. You should have around 5000 tons of cargo remaining (plus the tonnage set aside for the occupants of the Mech and ASF bays). Use that cargo for spare parts, food, life support, cargo (to simulate being a roving trader), and ammunition. Even then, the players should be trying to use OmniMechs and a lot of energy weapons.
For the mission itself, they are going for an Objective raid (the Macuffin), but getting out will be tricky. If the players are smart, they will frame one empire, while taking an indirect route to get back home (head towards Terra, then change directions). So if they are working for Marik, taking something from Lyran space, they need to behave like Draconis, and head towards Terra on the way out. Near Terra, they change directions and go directly back home, talking to as few people as possible so Lyran forces blame the Draconis Combine for the raid.