I may be completely wrong but I think the 3430 is the biggest you can get a Kubota MMM for. The Kubota MMM is around $2750 and the MMM pto kit is less than $300.. Maybe someone else can give a better answer.
Kubota B2710, IH TD6-62 dozer with Drott 4n1 bucket loader
According to a price list I was given about a year ago, the MMM and mid-PTO kit are available for the L3830. MSRP on the MMM was $2950 and $300 for the PTO kit. Again this was before discounts.
The L3830 will accept a mid mount mower. My understanding is that you need to have turf tires on it for clearance. The Kubota 72" mower is quite expensive. I thought the price was close to $3000. The mid PTO can be added to the L30's tractors. As mentioned, list was about $300, I think you might do better.
That depends on the type of sale. To have mount it I'd be looking that at that as an additional cost. If you are able to pickit up and the dealer only has to supply you with a complete mower then you should have some room.