I agree with Harmony. I would make sure that there is insurance for the horses and ask for a copy of the certificates, that you yourself are covered by yours. I assume they would look to deep litter in a loose barn on straw. Are they going to need hay and straw on site and is there somewhere to store it? I would include the extra storage as part of the price.
As mentioned if you are only providing the building (and water) then things such as feed, bedding, turning out, grazing etc can be eliminated.
I would also draw up an agreement for how many horses you are willing to accept what you will/ will not provide;
Just the sheds/storage with no responsibility to the care or welfare of the animals.
I would make it clear that if there are any welfare concerns and the owners cannot be contacted you will call out the vet at their expense or report them to the respective authorities.
Where they can have a muck heap, that is does/does not include grazing.
The length of how long you are willing to rent the space out, the notice you wish them to give/ you to give.
That they must clear out the sheds at the end etc. (take some pictures of before).
It is not unreasonable to take a deposit in case you do get landed with anything unforeseen (including a dirty shed at the end).
Get 2 copies signed and dated, also put in a sentence that states they have read and fully understood the terms and conditions, you do not want them to say that they didn't know what they were signing.
It sounds a bit overwhelming but it is better to watch your own back. People are usually very nice until it goes wrong. I am a horse owner and livery and generally would not have my nose put out by signing an agreement if they are genuine people. If they do not wish to sign anything it maybe a good warning indicator to steer clear.
I hope if you go ahead that everything works out well for you.