Not counting land rental or fencing, BH works on a rule of thumb that a store lamb will need £10 spending on it on top of its purchase price. This includes all commissions, levies and transport costs for both the purchase and the sale, any feed, meds, etc. (This is selling the lambs through the ring or deadweight to a supermarket.) You'd be unlucky to lose one or have a large vet bill (if you get one you often get the other too - and then you can add disposal charges to the costs, too) but of course it can happen.
Store lamb prices are very high this year, at these prices you only need to lose one to wipe out any profit and then some.
I've heard now of a few people getting a very decent price for Texel cross lambs out of Shetland ewes. Buyers of such lambs don't care two hoots whether the mum was registered, so that sounds like a good place to start. But for sure they need to be ewes who've lambed before if you're going to put a Texel on them.
If you plan to lamb yourself, you'll need to think about any facilities you may need - you say the field has no buildings, so you would need some sort of field shelter for any problem families and orphan lambs, or be able to move them to somewhere with facilities for them.
Sorry that's all rather random scraps - but hopefully of some use anyway.
Enjoy the planning and choosing - it's an exciting opportunity!