You'll get a second hand compact tractor and flail for less than the cost of the loader flail.
I don't know why people insist on using a loader for tasks that could be acheived with a compact tractor. A hydraulic PTO is so inneficient, and 3-pt linkage attachments are way, way cheaper. Even secondhand, an Avant flail commands strong money due to rarity- probably £2.5k You can pick up tractor flails for £1k all day long.
My Kubota compact has been a great purchase, way more useful than I dreamed. In the last two years I have bought:
Flail collector- £800
Second flail collector plus scarifier blades- £1200
Vacuum grass collector to pick up from the mid mount deck- £250
PTO driven deep aerator- £500
Topper (new)- £800
Stump grinder (new)- £1800
Mole plough- £400
Firewood processor- £4500
Heavy roller- £250
Fertiliser/seed spreader- £250
PTO generator- £400
Roller fed chipper (new)- £2500
All those attachments are available to fit a loader, but you'd be very lucky to find them second hand and they would cost five to ten times as much new (really not an exaggeration, £8k for the Avant flail collector!) I can provide a huge range of services with gear that cost me peanuts. A loader is to load and carry (and they are great at it). A tractor is to run implements requiring power efficiently.
You won't run any sort of flail on a Sherpa or Cast. Even the 22hp option only puts out 28l/min. A 1.5t mini digger puts out 30l/min, some claim that their 1.5t runs a flail great but lets be honest, they don't really. They can only slew- any other movement robs all the flow. You need very expensive piston motors on the flail to get even a reasonable performance. As the little loaders have hydraulic drive, you simply won't be able to drive and flail, it'll be like tracking and flailing on a mini digger which is widely acknowledged as impossible.
In your circumstances, I appreciate that a compact tractor won't work (width). I'd get yourself an engine driven flail like they use behind or in front of quads, and fit loader brackets to it. That would be very effective. You could also consider a two-wheel tractor with flail mower, or a Scag/Ferris self propelled pedestrian flail.