Squarespace Customisation Capabilities

I would say that all of these are achievable on Squarespace but they would require third party software. 

I’m not sure which inventory management system you are using, if they have a direct integration with Squarespace or if you would need to bypass the Squarespace commerce system entirely. If they allow it then you may just be able to add some sort of widget to your Squarespace website.

As for the logins I would look at something like Memberspace. it will cost extra but it will do what you need it to. 

https://www.memberspace.com/

If you want to use Squarespace’s built in customer accounts to gate content then there is the plugin by SquareStudio but your content won’t be secure. 

https://squarestud.io/collections/plugins/products/subscription-paywall

For users uploading their own photos I would use the upload tool created by @michaeleparkour.

https://www.google.co.uk/amp/s/www.squarewebsites.org/blog/squarespace-websites-uploader%3fformat=amp

I would suggest trying to find a developer from this forum rather than marketplace. 

If you hire someone from marketplace a large percentage of the fee goes to 99Designs (who host the marketplace) whereas a developer hired from the forums will get the full fee.

Be nice - don’t give 99Designs a slice. 

You'll also find that the best designers and developers have not signed up for 99 Designs. 

Both @colin.irwin and @siluntz are correct here—the vast majority of the most experienced Squarespace experts who were on the original Squarespace Specialists did not join the 99designs scheme for many reasons, including the unethical business practices of 99d. I wrote about this here: https://sarahmoon.net/blog/regarding-the-replacement-of-the-squarespace-specialists-list-with-99designs

But to answer your original questions, while you can do what you want to do on Squarespace, albeit using a lot of custom coding and third party applications, your needs aren’t a great use case for the platform. I’d definitely research alternatives before committing to a Squarespace development process.