Thanks Sally - tried to reply by quoting you so I could go through point by point and the formatting turned into weird columns, so here's try number two...
Are there any costs associated with the land? Rental, fencing, etc? - £2 a year per share common grazings fee, so £10-£12 annually. One fence needs some attention, but the other side is Billy and between us we can fix it up easily enough.
With that many ewes you should budget some vet fees, I'd think. And antibiotics, flystrike prevention, flukicide, minerals, foot spray, etc. - good point. I'd mentally covered flukicide under dosing and minerals under Crystalyx and boluses, but hadn't thought about antibiotics. Flystrike very rare here due to altitude, but yes, they should get a fly and tick dose after shearing.
I assume you don't have a day job? So could manage the lambing yourself? - correct, I freelance from home.
You might need labour for shearing, if you're getting shearers in. - should be okay, we have a communal shearing in the village fanks, so all I need to do is gather them and then we all help out with pushing the sheep through and rolling fleeces.
Might you need an accountant, they cost a few hundred. - already got one
If you want to maintain your 150 breeders, and each lambs 3 times before being drafted, you'll need a few more than 50 retained ewe lambs each year, to allow for losses. - very good point, black loss on the hill can be high in a hard winter.
Dead cart / disposals are another cost. - I'm classed as a remote area, so it's all on-farm burials.
Not all of your ewe lambs will be worth £55, and not all your draftees £100. - true, but calculating on mart averages is the best estimate I can make. Some years it'll be up, some it'll be down.
How much will you have to pay for the tups you buy in? In general, anyone selling good tups for £300+ will be seen to be buying in good bloodlines - perhaps spending £1000 or more on a top tup every now and again, and probably £500 plus for the others? - my plan is to go to the big tup sale in the autumn and look for an aged tup with great bloodlines for around the £750-£1,000 mark to use for two years. As the flock size increases, I'll buy younger - at the full 150 ewes I'd be looking at 3 tups for around £1,500-£2,000 each, so budget for one or two newcomers each year less whatever I get for the outgoing ones.
Do you always get hay weather? Or might you need to make silage some years? Plastic and wrapping are expensive. - pretty much. I've lived here 9 years now and there hasn't been a year yet that my neighbours haven't managed to get hay in, though some years it's been a case of holding your nerve until the end of August.
Have you got somewhere to store all the hay? Enough buildings for lambing 150? (I know you'll not be lambing indoors, but you'll inevitably need some pens for some ewes and their lambs.) - ah, I probably should have mentioned that the 18 acres also comes with a house, barn, four stables, dog kennels, lambing pens, small fanks and outdoor school. It's next door to us and their sale fell through yesterday, so we thought since we were looking at spending the sharp end of £40,000 this summer on putting up a barn in the big field we own on the other side of it, why not see if we can get a mortgage and buy that instead? The plan is to move into it, give ours a makeover and then our house becomes our second holiday let.