A couple of thoughts...
A BX is NOT a B in any way, shape or form. It's a lot less tractor, plain and simple. (This coming from a guy with three B-series machines.) You would be making a huge step DOWN in performance and durability.
The B7800 is a good core machine. Don't be worried at 1300 hours - I have a B2410 with 2600 hours and it's running strong. I've replaced the dynamo, that's it. Do the scheduled maintenance a little ahead of schedule and the core tractor will serve you well.
As for implements, if you are mowing 5 acres do not sacrifice one inch of cutting width. I find the difference between my 54" and 60" mowing decks is considerable when mowing at the airport. I use the narrow mower when the grass is long or wet, otherwise go to the wider mower and mow with my foot to the floor.
You haven't stated how much snow blowing you do. Given your reference to being in the fourth quarter of the game, a front mounted blower makes all the difference in the world (I have three of them, plus hold a rear-mount blower in reserve, plus have another bigger tractor with a rear-mount blower just in case - we get serious snow here).
If I were in your shoes I would likely spend the cash to refurbish your existing implements, replacing bearings etc, or look for the same model type but with fewer hours on the used market. Mowing decks aren't hard to find. Plus if your mowing deck blows up you know you can always just buy a stand-alone 3pth-mounted finishing mower. They aren't nearly as nice to use as the belly-mount mower if mowing fairly flat land, or at least that's been my experience, but they are relatively cheap, readily available and adaptable to any tractor with the right PTO horsepower.
As for the tractor - just spend the bucks on a good once-over for maintenance. Inspect, repair and replace as necessary. It might cost you a couple of thousand bucks to do it well. The B7800 is a terrific platform which, at 1300 hours, isn't even close to mid-life. None of the newer tractors are as tough or as heavy-duty as your existing machine. My neighbor has a B7800 that he has considered trading for a newer tractor. He came home shaking his head at the dollar difference the dealer wanted. I've already told him I'll buy his tractor for cash when he wants to sell it as that generation of B-series is a terrific machine.