I would say take the time and expense and have level ground. Otherwise you'll be forever and always fighting a slope and water.

We built the greenhouse (18' x 28') and an attached garden house (20' x 16') on the slope where the bank barn used to be (high winds blew it down in 2000)) My husband turned that site into a nice, gentle slope, not knowing we would want to build there again.

To make the greenhouse level, we had the site shaped as much as we could, then brought in tons of limestone to make the whole site the same elevation.

Once the greenhouse/building were built, brought in fill dirt to cover the stone and build up a bank around the greenhouse (the spot where the garden house is was level and we made the greenhouse meet it)

so glad we did it this way.
Best of luck on your venture.

Thanks for the comment. I called the backhoe guy out today. The prices were really high for bringing out gravel/limestone, but leveling it with our own dirt isn't too bad, and it seems doing it right, now, is the best thing.

Do it right the first time. It'll save a lot of grief and expense later.