What themes and plugins are needed/compatible for a catalogue-type site with limited merchant functi

Hello,
I have been reading for the last 5 days and I have several questions. Here is a description of what I would like to set up:
-A main product catalogue on the main domain/page: sellers list their products, customers look for products.
- Sellers can: have a slightly customizable blog (can it be only colors and images? For example Teatrade and Fondelifair are bit too messy for me, where everyone can do whatever they want, I prefer the Etsy look). Each seller handles their sale and merchant options individually; I want nothing to do with it.
- A searchable directory of all sellers and a searchable database of products (advanced search)
- A network wide forum (not per site) for questions or suggestions on new or existing products and services and their uses.

So here are my Questions:
1- So, what theme should I use? I think it is Frame-Market with child Gridmarket? Or should it be Product? Business Portfolio? Blogs MU? And I can't locate for each theme what plugins it already include, is that info available?

2- In order to have a network wide forum not a forum on each site), do I need Buddypress or bbpress plugin will do? Or is there something else? After reading here and on the web It is still not completely clear to me what is the difference.

3- I am confused about the essential plugins I will need, it seems to me there are different plugins that cover the same functions. I have looked over all of them (I think!), and I have come up with a list of plugins that I think I will need, so are the following plugins appropriate/compatible/needed for the theme you will be suggesting? Do you recommend any others that would be crucial for it to work?
- Pro-Sites or Pay to Blog?: to administer seller membership types
- Directory or Members Directory or Blogs Directory, or Users Widget?: So visitors can see a list of sellers and or look them up by location, type of products, etc.
- Moderation or Monitor?: to monitor inappropriate posts and comments
- Market Press: to enable merchant options? As far as I can see it is included in Frame Market, rigth?
- Post indexer, Global Site Search or Global Tags Search: to index products, so visitors can search items using different criteria, something like and "advanced search" feature
- CustomPress: so I can customize fields available for product listing
- Avatars: so sellers can include their picture in each product posting.
(There are other plugins but I think its clear to me how I can integrate them...we'll see)

4-And just to confirm: either if I use directory or sub-domain format for my network it basically has the same result: sellers can create their own sites

5- Can user capabilites to add/edit/delete/deactivate networkwide categories, plugins and widgets be deactivated so site admins can't alter these components? OR when defining Users, can a new type of user be defined, something in between an "Editor" and an "Author"? Someone who can do everything except alter these features? Does Pro Site offer this capability?

I hope I am not abusing the service I have tried to chat with someone, but I don't seem to have the times right, I have searched, but have found partial/old/unclear responses and some of my questions I think are too specific.

Thanks in advance for any advice!

Glenda