I don't need a new tractor or one bigger than I already have, but if I ever considered a trade up, the only one that comes to mind is the 3030HSTC.
If I was in your situation I would hear what
Lawn Boy has to say. He has owned a bunch of cab tractors. Number two lives in an area where he has used it when the temps are way down there and the snow is way up there! He makes a living sitting on his tractor and has a bunch of hours inside the cab.
I believe for aftermarket the Curtis Cab has a good name. If one buys the soft side cab the doors come off easily for summer time use. You can get an optional heater and driving lights and I am not sure what else Curtis might have for the aftermarket cabs. The hard side cab doors are much nicer with sliding windows which work nicely if you want to talk to someone, at least you do not have to fling the door open!
Without the heater it will be in the 2000 to 2500 range then you will either have to install it yourself or have someone do it for you.
There are add on cabs that look more like golf cart covers than functioning tractor cabs and some of them sell for as high as $1600.
I have a Femco Weather break with a sun shade type top which works to keep the snow off from me and the heat from the motor does come up into the cab, but with that said it certainly is a poor man's approach to a cab. I am not sure how much I have invested in it, but it would approach $800.
I am looking at a Curtis Cab retrofit for my B-7500 right now and the pricing for a soft sided deluxe is $1500. The hard side is around $2500 (this has the nicer doors and I think better back window shield, but I am not sure) With either the hard or soft side cab you can add a heater unit and also the lights offered by Curtis.
Here is a link to the page I saw the pricing:
Curtis Tractor Cabs for Kubota Compact Tractors