I think that the Mahindra 35 series has been discontinued, I just bought the 5145 it is a cross between the 36 series (replaced the 35 i think and has also been discontinued) the 55 series and the old school 45 series. It is heavy, close to 7000# with FEL, the FEL will lift 3100# at the pins to full height, it has a 12f 12r shuttle (no hydrostat) it is built like a tank and the cost is verrrrry good. The 51 series is more basic with no electric over hydr stuff not power shuttle so the price is less than the 2660 and less complicated for stuff to go wrong. the 51 series is the largest tractor that Mahindra makes that is still considered a CUT so it is still small enough and maneuverable enough to get into tight spaces.
I stuck with the 45hp 5145 version because it is $4k cheaper than the 5155 and does not have a turbo. The 45 would be good for most of the things you are listing that you want to do but I would think that the 5155 would be better for you, mostly the mowing large areas of pasture, my dealer tells me that the 45 is marginal when you go up to a 12' rotary cutter but with 80 acres to mow you might need the extra hp / cutter size. The 51 series is a very heavy and stable frame with a solid drive line.
As far as pulling some thing out of the mud hp is not the answer, weight and traction are, so you may want agg tires not the r4 industrial tires and a heavy 4wd tractor. It looks like you have the equipment to haul a 7K tractor so IMO go big on frame size and Mahindra will give you lots of that.
I have no experience with the backhoe, so I will let others fill you in on that.
I spent a lot of time researching everything that was available in my area. JD and Kubota were about 2x the price for what i was wanting, TYM, Koti, Branson all had competitive offerings but no real dealers in my area, LS has the MT342 which was my second choice but the engine is a turbo which I wanted to avoid, the dealer was 100 miles away (but a nice guy) the killer was that LS will not allow a customer access to the service manual (at all even tried to get the dealer to make some kind of a deal but no go, I had the factory Mahindra service manual before i got the the 5145), that was too bad because the MT342 would have been at least $5k cheaper for a tractor with all most as much capability (LS has a very strong offering with lots of premium features for less money).
The best for last, IMO the most important thing is how the tractor and the dealer fit you, so spend some seat time on the tractor you think you want and talk with the dealer, I spent a few more dollars going with my local dealer instead of the big dealer that is 120 miles away mostly because the local dealer did not act like a used car salesman. Also the tractor i ended up with, I love but it was not even the tractor i was looking at until i sat on some.
The 51 series is new (may be a con but most of the assembly's are carried over from the models it is replacing) here is a thread on it:
Mahindra 5155 Anyone?
Good Luck and i hope that i have been some help.