One idea is to start it as a skunkworks project, with a very experienced and skilled person starting solo.
With the express understanding that, if this is successful, you'd expect them to ultimately lead it (head of engineering, R&D, CTO, or whatever fits).
Greenfield development is appealing, and the big growth potential adds incentive to do that greenfield in a way that's aligned with the goals of the company.
Don't make it super-lucrative initially, to help weed out the serial job-hoppers and the transactional hours-billers who aren't as invested in the long-term success.
On your end, you only need buy-in that, if this succeeds, then company will want to follow through.
With that understanding, it's only a single hire to justify.
If, when you review every 3 months, it's not looking like it will work out, start over with a new champion. Your only lead time is to find one candidate who you're willing to give a shot at it.
It's their responsibility to make the skunkworks so successful that the company is confident in taking the next steps of greater investment.
A complementary possibility to keep in mind is that, if you really execute well on this system, maybe you could spin it off into a subsidiary that provides IT solutions to other companies in your field. (Especially since your own purchasing experience sounds like there's market opportunity.)