Welcome to the forum!
Like stated, the 1811 is one of the best GT's Cub Cadet ever made. Second only to a Super Garden tractor, or maybe an 1864 and that's only because it (1811) doesn't have power steering. (And you will wish it did with that blower on it!
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I have ran that combo for years. Great set up. You are going to wish you had a cab after blowing a lot of snow with it though.
You got a really nice machine. Looks to be in good shape! The blower looks as if it hasn't ever really been used.
As far as the leak goes.... It wouldn't take long to pull the driveshaft and replace the seal. Is it a common issue? Well..... it's a 30 year old machine. I've replaced the seal in mine at least once, maybe twice in the 15 or so years I've had it. There are some other places that are more common to leak, such as the trunion seals, cork gasket and relief valves. I'd probably take an afternoon and fix it. Taking the shaft out is not terribly easy on that tractor. Either the rear or the engine has to be unbolted and slid back a bit to get the shaft out. For what you have to fix, I'd probably loosen the engine plate. No reason to take the rear loose unless you need to.
As a note: I'd be changing at least the filter on the hydro. I can see it in the pic. They haven't used that logo is quite some time. May even be the original filter. Makes me wonder if the oil in the rear has ever been changed. I'd change both oil and filter. A Wix 1410 or Napa 51410 is the filter you need.