With apologies, I'll post a half-answer, because I'm helping a relative with a move and I don't have access to my manuals (and some of my brain cells, lol.)

Over the years I can recall another HFGH owner years ago, contacting me through my blog, and they didn't want to use the steel base for the 10x12 either. Their concern was a very wet environment; they felt the steel might not hold up (although I don't know of a steel base ever failing; they're pretty heavy.) I know they were successful with their build, but I don't know the specific re-engineering steps that eliminating the base may have caused. So it's possible to eliminate the base, but if there are resulting tweaks I can't outline them for you.

You might specifically check to see if the part 47 hold-down connectors (that secure the diagonal braces to the base) attach to a lip on the base itself, or to a lip on the top plates. (It's been too many years since my build, and I can't recall...and don't have easy access right now.) I just have a question in my mind about that possible complication.

In my warm climate I didn't mind the extra inches of height that are contributed by the base, even though we mounted our steel base on top of a (taller) wooden foundation as well. My biggest problem is cooling, and warm air rises, so a slightly taller greenhouse was a slight advantage for me. It did require me to build a few paver steps up to the doorway on the outside of my greenhouse, and there's a very slight step down to my paver floor inside.

Apologies for the half-answer! Maybe someone who has built one more currently could do a little brainstorming with you. I do know these kits lend themselves well to re-inventing, if you're willing to figure out the possible solutions. Over the years I've been very impressed by changes that are possible with a little creativity.

Thanks so much for the prompt reply. I'll keep diggin through the instructions to see what I can figure out. I will probably end up using the base and adding a step up to the door as you mentioned.