I'm assuming the £1500 a month is after expenses? Bear in mind you'll have additional water costs and electricity costs (running a business, so no energy price cap). There will also be costs for replacing breakages/damage etc and probably some maintenance will be needed every winter to keep shepherds huts/bell tents looking fresh. So you'll obviously need to be making a lot more than that in order to have that as your left over income.
£120 a night is pretty ambitious for a Shepherd's hut- around here (also south west) you are probably looking at £50-70 a night for a Shepherd's hut that sleeps 2. Bell tents are normally in a similar price range, but might sleep a whole family. £120 a night for this sort of thing is really high end, so would probably require lots of investment at the start.
Kitting out a shepherd's hut or bell tent to a high standard will require some investment too- as will putting in a shower block or similar on the land!
Realistically for bell tents, your season is around April to October, but if it's a wet summer like this one, you won't be fully booked all the time. For Shepherd's huts, if they're well insulated (and so you've spent money on a decent one) then you can sleep in them all year round, but not that many people will be booking them in November/January, for example.
Really you're probably looking at needing to make £3000 profit for 6 months of the year- which is probably going to be a bigger concern than you're imagining, and comes with lots of complexities.
If you're going to buy somewhere with land, and you'll be near Dartmoor, for example, you may be better off just having a traditional campsite- I know it's not as fancy, but it'll be easier to run and people won't expect such high standards from you! You can also get a bit of off season stuff in the spring from ten tors etc, if you're willing to accept several tents of teenagers overnight at weekends!