If the tractor is going to be a mowing tractor with light FEL use, then my choice would be the Kubota. It has 24hp versus 20 (Mahindra) & 21 (Kioti) hp and that will yield about 3 more PTO horsepower which will give you more power to run a wider mowing deck. A 60" finish mower is absolutely no problem with a 24hp tractor in very lush thick grass, which gives you a bit more reserve power than you'd have with the two other choices. I know the B7610 will handle a Mid Mount Mower. I believe the CK20 will do the same. I don't know about the 2015 so I have to plead ignorance on that. Obviously all 3 would run a rear mount finish mower. But anyway you add it up, no matter what size mower deck someone says either the Mahindra or the Kioti can handle, the Kubota, with 3 more HP, can handle your choice of either a larger deck in identical conditions, or will not bog down in tougher conditions with an identical sized deck. EDIT: I went back to the Mahindra site, the 2015 does offer a MMM deck.
The FEL is somewhat lighter duty on the B7610 than the Kioti, and I prefer the Kioti's loader, but you said it was light duty work anyway so the advantage of the stronger FEL is irrelevant and certainly not enough to offset the lower PTO hp.
A heavier tractor will do better with a box blade than a lighter tractor, but realistically all three tractors are well suited to a 48" box blade and nothing more than that in any compacted soil. So I would hazard a guess that all 3 would be just about equal with a box blade.
As for the tiller, I favor light tractors for tilling a garden largely because I don't want compacted soil in my garden. A heavy tractor offers no advantage to a small gardener, but certainly does to a real farmer. Also, the extra 3+ PTO hp of the B7610 will power a slightly larger tiller and/or will give you reserve power so you can till at full depth on the first pass if you properly size the tiller. Again, as with the mower deck, there is no substitute for the added 3+HP that you gain with the 7610.
Now my choice is really based on the fact that as I see your tasks, PTO hp seems to be the deciding factor because of the implements that need PTO connection, and also because you said the loader work would be moderate. Any of the 3 will lift a bucket of dirt, but if I recall the specs correctly, the 7610 has the lowest capacity of the 3.
I'm sure you'll get other opinions, but unless they are the same as mine, then they will be wrong! Of course, if they are the same as mine, they are obviously by enlighted posters. /forums/images/graemlins/wink.gif