I think that I have found both :-)
Let's hope so!

- It seems that I can create custom fields e.g. Item Manufacturer is it possible to allow people posting new items to "add" new Item Manufacturer on the fly?
Yes, this is a feature called "click2add" and it can be set individually for select fields
- Would it be possible to associate a weblink with the Item Manufacturer so that on all the reviews for their items it showed the weblink to the Item Manufacturer (without having to enter this each time).
It's not possible to define field relationships right now, so this is not possible.
- Can you tell me more about the messageboard plugin? Does this allow you to start a thread to discuss an item?
You don't need the jReviews JB addon if you just want to allow starting a thread to discuss an item. There's already a free joomlaboard discuss bot for that. With the jReviews addon you can do that as well, but you can also:
1. Associate jReviews categories with forum boards, so the threads are started in corresponding boards and not all in the same board as it happens with the free bot
2. Allow discussions on reviews, and not just the items
3. Create new forum posts automatically when a new item is submitted. This helps populate your forum with posts related to those items and that post serves as the first post for the item discussions.
- Would it be possible for me to create simple joomla page and this would dynaimically display the top 5 products from each category? Or other similar flexibility?
There's a post ion the tips and tricks board on how to load a module into a joomla page. You can use the dashboard module in that page to achieve what you want.
Also I would like to know what your rate is for custom work...$$$ Thanks,
I don't have a published rate and would rather discuss this in private. I don't do all the work that comes to me because I don't have time. Especially now while I work on the new release, but I am open to hear your requirements and if there's something that the overall jReviews user base can benefit from I might give it priority over other todo's.