If they want to purchase in 18months, sign normal lease now and tell them to make you an offer in 18months.
They make you the right offer then, you will sell to them then.
No funny business in the meantime.
How do you evict an LLC if they don't pay. How do you check credit on a new LLC.
You want individual names who are responsible and who have a stake in the lease and the consequences that come with it if they default or don't pay.
Keep it on the up and up.