I was you.
First of all, don't give a second's thought to the fact that you've never snowboarded. I spent the morning on a softboot setup getting a lesson from my hardboot-riding instructor friend and hated it. In the afternoon I put my ski boots back on, switched to his board, and never looked back. Coming from skiing, riding a hardboot setup with forward-facing angles felt so much more biomechanically correct to me. By the end of the weekend I had bought a used board from a consignment shop. My racing boots were really too stiff for snowboarding but I used them anyway for a dozen or so days before springing for a pair of snowboard hardboots.
I think it's a great idea. Are ski boots ideal? No. Maybe. Sounds to me that getting into the sport at minimal cost is your definition of ideal for now. Some ski boots will be better than others. Can you get away with it for a little while? Absolutely. Will it hinder your progression in the sport? Not if you're aware of the issues, keep them in check, and adjust when you're ready.
Look for a bail-type binding. (Your ski boots won't be compatible with step-in models.) Keep in mind that bindings play a role in flex as well and a flexier binding is going to be more forgiving of user error. If you're in Lange RS130s on TDs/TD2s/TD3s/Cateks life will be a bit harder starting out than if you're in Full Tilt Classics on Race Plates/F2s/TDSWs.