Not too many luxury items,, maybe a bathroom would be one?

Pretty straight forward needs thou, 

1. elevation get it above the surrounding area *higher is better*

2. rock, 2-3' deep, 6" gravel over that

3. heavy plastic sheeting over that

4. 2-3" gravel over that

5. either asphalt or cement over that

6. drains under rock if you really have ground water problem (french drain works)

7. 2-3' overhang on roof line to get snow/water away from footings/base

8. siding on least 3 if not 4 sides.  (heavy canvas curtain like a shower curtain
   on front will help)

9. least insulate underside of roof

10. Electricity for motors/lights/fans. *might be* luxury but least get juice into barn

Height is cheap. don't be skimpy, might be using a telehandler or squeeze fork lift in the future,, so it'll be easy to stack high. (something like this)
 

*here* most all hay is moved with squeezes like this, hardly anyone handles hay by hand.  (slickest way to fill barns-load out trucks)

May sound like "overkill" but it's no different than "underbuilding" a grain setup. Finding out later "man I wish I'd done..."  (If you wind up loosing the bottom's, or sides, or tops,, whole lot of money spent and still not right- alot harder to fix after built)

It's common here with a good 1000 ton barn to not have 1 bale rejected for spoilage.

The other benefit is you'll have good machine storage when the barn's empty.

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