Why lower or raise rent?
If they want to pay for it and use a contractor that YOU approve, AND:
1) they pay for it and reliquish it to you when they leave at no cost to you with no reimbursement to them whatsoever (and SIGN an agreement stating as such... absolutely must sign such an agreement)
2) AND they sign a waiver making you not liable for any injuries resulting from the use, install, etc. of the storm shelter.... (an absolute must to waive yourself of the liability)
.... then you might just have yourself a free storm shelter.
Make sure they sign both a liability waiver, and an "improvements/leave the shelter with the house for free" agreement. And use a reputable, insured, licensed installer that you approve.
I would not buy a storm shelter for them. Are you going to start buying everythign else they ask for? If they want granite countertops and a swimming pool, are you going to buy them those things too and charge $25 more? They want a luxury item and are willing to pay for it/sign waivers/leave it with the property/use a contractor (licensed and insured) that I approve, I'm in.
I would also use caution in that if you ever anger them or raise rent or anything else, they'll dangle this carrot over you forever. You try to raise rent and they'll say "but we bought you a storm shelter." They want a cat and you say no cats, they'll say "but we bought you a storm shelter." They pay rent late and you charge a late fee, they'll (again" say "but we bought you a storm shelter."
If they're good tenants with great references and sign both #1 and #2 above, I would probably agree to it. I mean, a "free" storm shelter. But, obviously, if anything ever goes wrong or you raise rent or they pay late, they'll hold this over your head forever. Personally, I would probably take this risk in this situation so long as they do all the above and sign all the above.
Also keep in mind that they need to make sure the storm shelter is installed property, doesn't flood or cause flooding, doesn't hit a natural gas or plumping pipe, etc.