I work in bike hire (and transport) in Inverness and I don’t think I’d even go near this 🙂
The ACT and CTC have Bike Hire Guidelines documents you might want to read. They’ll give you some idea of what might be involved.
https://www.actsmart.biz/uploaded_files/cycles-files/C3.%20Hire%20guidelines.pdf
http://www.cyclinguk.org/sites/default/files/ctc_hire_standard.pdf
I’d be very surprised if any manufacturer will give you a “deal” and AFAIK On One are pulling out of fat bikes anyway (the market has peaked apparently).
I’d also advise against acquiring a rag-bag of makes and models. Fatbikes standards are all over the place with hub widths, front hub disk spacing, BB lengths, crank spacing etc. You’d want to have everything the same. Have a look at the Calibre Dune or the Voodoo Wazoo if you want cheap/decent bikes but be prepared for a high maintenance cost – especially given folk are bound to end up in the sea with them.
You’ll want a range of sizes. You need to work out what your market is. Mostly, I suspect it would be non-cyclists looking for a wee spin. Once they work out that pedalling on soft sand is really hard work, they might not get very far. Also work out the safety issue – are there spits/coves etc that folk could get to and would land them in trouble?