Building a CMS isn't lean. Using an off-the-shelf CMS that is selling enough of your photographers' work that both you, the end customer and the photographers all are saying isn't working anymore because of X, Y and Z reasons is lean.
Then, if you're at that point, I'd start looking at hiring a full-time dev to build a product infrastructure that's going to scale you up, instead of outsourcing.
I've been involved in too many "we'll just build the product with a custom version of an existing CMS" and also the "we're going to build everything from scratch before knowing what anyone really actually needs" so hope you can avoid some mistakes I've seen and felt pain for!
Happy to talk if I can be of help.
I think Wordpress is definitely your best answer and I would be glad to show you how easy it is.
Photo Gallery - Photographers can easily upload their work:
With a few Wordpress plugins (which are easy to use), there are plenty of photo galleries out there for your photographers.
Photographers selling their work:
I can show you how to add a simply ecommerce plugin on Wordpress to easily help the photographers sell their work online.
I would love to show you how on a call.