Hello,
I'm also of the view that you need to have a chat with the landowner, and see if they are open to the idea.
If they are, then do they want to rent or sell ?
If they want to rent, then as Mr. Burgess correctly points out, commercial leases typically include a provision for a rent review, every 5 years, to include how the new rent will be agreed (and where it's not agreed, that it goes to mediation and who might meditate).
If you are considering putting in €100k, how long will it take you to get that money back, and thereafter, a profit ? That's the minimum term of lease that you should be looking for, though preferably a longer term than that.
You may need to pay vat on the rent, depending on term of the lease.
You will also need to deal with such things as who is liable, in the event of an accident, while someone is on the farmer's land - so insurance, to the property owners satisfaction, is a must.
Will you need planning permission for the project, either for general use, or any specific construction that's needed ?
I would suggest having a chat with an accountant, preparing a business plan, to include some financial projections, before speaking with the property owner. If the landowner has an interest, then you may need to share part of this plan with them, albeit I wouldn't share the financial forcasts, as the better they look, the higher the likely rent that the farmer will request.