The main difference, as pointed out, between the 2320 and 2520 is direct injection on the 2520. I've read that this is a bit more efficient. It's faster starting only if you forget to glow the 2320 (4 seconds on the coldest day). Think this is available on the 2520 at a lower price increment than was between the 4110 and 4115, for instance.
A rear blade with some skids to keep the blade off the gravel would be cheaper than a quick tach front snow plow. The BB and FEL make a great combo for clearing snow.
The real seller for me among the choices you're looking at is the position control. Wish I had it on my 4010. I'm forever looking back there and guessing where to drop the implement.
Another thing I've seen is the hood folds back and makes access to EVERTHING, incl. the battery, so much nicer than the older models and better than the older Kubotas, too.
If you have any uneven ground at all or want to do much brush hogging, scratch the 2305. That underneath plastic hydraulic cooling fan is very vulnerable. The hydraulic cooler on the 2320/2520 is like it was on the 4000 series: in front of the radiator and uses the engine cooling fan.
Turf tires work fine on my 4010, even in our recent sleet storm. Don't think it slipped at all on the driveway with the wheels behind either the BB or the FEL. Did slip some on top the sleet. Even plowed some of the road uphill from our cul-de-sac. Still no slip. Turfs are mainly not very good on mud. About the best for mud is R1s, but they may dig you into a hole.