A couple of thoughts on your BX purchase. Unless you need the width, 54 inch decks are lighter and easier to get on and off. 60 inch decks are pretty heavy and will need to be taken off on a cement floor so you can roll it out, if you don't want to struggle. Having a loader on the tractor to lift the front will help.
Higher HP is something you won't regret especially if you want to run a leaf collector. Kubota makes an excellent 3 bag collection system for the BX series. Usually you can find them used for sale online in good condition. They have a very good blower.
A mulching kit is another option unless you have something like heavy oak leaves. I run a factory mulch kit on a ZG227-54 zero turn mower. I have run it for 12 years on my lawn and mulch all leaves. After 13 years and no fertilizer, the lawn is much greener with more grass and fewer weeds. The Kubota mulch kit is a permanent install. Have to pull the deck and change baffles, as well as change blades to go back to regular. If you mow a wet lawn, the mulching deck will plug up completely in about 20 minutes and is a bear to clean out. So I tend to stay with mulching and work with the weather. You will definitely want the higher HP to run the mulch deck.
As others have commented, the two fuel filters with one directly over the mower makes the fuel filter set up not idea. Two speed hydro limits road speed a bit. I do most mowing and loader work in range 1.
I have an L35TLB, too. Used in business, 4500 hours so far. Always used Kubota UDT, which is not synthetic. I don't want to switch to synthetic SUDT now, so I use a high quality name brand equivalent which labels itself as meeting UDT specs. Engine oil for the
L35 has always been Shell Rotella 15-40. The BX has gotten UDT, too. SUDT is a little thin for a machine built to run non synthetic UDT. But a new machine might do fine with SUDT.
I don't know the engine oil spec for the new BX series. If it is 10-30, I would run 10-40 synthetic to give you some margin if you work it hard. I change oil yearly or at 150 hours, a little earlier than the specs.