Image Library for Stock images

I am building a website that serves as a stock image library site where users can browse through the thumbnails available, and once logged in – they will be able to download the hi-res versions. This is going to be a free service and I am building it on wordpress.

Anyone hear or used a good solution to this, like a plugin that manages your image library. I have run some tests with a few plugins and so far like what I see with Next Gen Gallery plugin. The only thing is I want to check if there isn’t a better way I can do this as I would like some functionality of the following:

• Automatic unique file naming. (Like 00123456)
• User management privelages
• Monitor Downloads of images
• Thumbnail generation as well as different size generation
• Add captions and descriptions to images
• These must be searchable
• Display them via categories

I know this is alot to expect, but I was just checking – hoping ??

Any advise or thoughts? Thanks in advance.