Creating a large website on WordPress

[smartblogger]

I'm creating a website on WordPress that will have 1000s of pages. The website is essentially a catalogue. Can anyone tell me:

1.The best catalogue plugins that do not have to use prices. And can add multiple filters like a shop to select 'colour' or 'size' or 'range' in a search menu.

2. I want to upload all of my catalogue information via Csv so that it automatically populates 3 different Web pages or more. Is this possible?

3. Advise which theme is better Avada or Genesis? As I've heard conflicting information about which one is more secure.

4. Advise on areas I need to take into consideration when building a site with many pages.