The specs on the ebay kit suggests that it needs a 140mm dropout, likely too wide for your bike. It also uses a freewheel, so cassette users will have to settle for a less sturdy set of gears. I bet it will fly a bike at 33-36 mph, speeds too high for most rider keep up a pedal cadence. I can only guess at the energy consumption above 30 mph, Maybe 40-50 wh/mile? The 72V battery is 1300 watt-hours (wh), so an optimistic 26-30 mile range at those speeds. For a bike enthusiast, is 30+ sustained a reasonable speed? Is it necessary? Your call.

The Bafang HD will probably roll along all day at 30 mph too. The less expensive BBS02 is more like 26-28 mph. Luna sells you a lot of stuff you don't need. Many owners, like me, spent less than $1000 for a BBS02/battery and are happy. Last I read, the current Bafang middrive kits lost their ability to smoothly mix throttle and pedal assist. If true, I wouldn't want one. What I like about my BBS02 is that it's virtually silent and adds little drag. I've pedalled it w/o power for 20 miles. Well, I can do that with all my ebikes these days, and when I started, I could barely pedal a regular bike that far,

If budget doesn't matter, better to have both the old bike and an e-bike. They are somewhat different. Your old bike is light and maneuverable, with less concerns. The ebikes tend to be heavier handling and heavier period.

Moving to a fatbike is quite a change. Mine is a low end steel frame bike that I converted to electric. I liken it to owning a Hummer in the suburbs. It draws attention. It's a ponderous machine and of little use on pavement. Knobby tires sound like a squadron of bees. I switched to smooth tires, but they seem a little squirrrely on corners, It's still fun. I rode in snow twice. Enough of that, One of these days I'll get it on a beach. It's gone as fast as my mid drive, but my preferred speed is lumbering along at 12 mph.
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