Martin as far as the categories are concern its always up to you but you should always keep the structure clean so that visitor can easily access to every category page without any problem and so as Google bot.
With URLs, it is always great to keep it short and easy to understand. In my opinion you should have to have a complete product name for product pages... that will look something like this...
http://www.domain.com/name-of-product
For categories, as you said you have 50 categories so if it is possible for you to divide in to like 5 major categories and then each major category contain 10 sub categories.... (if it is possible) this way the infra structure will be clean and easy for users as well as for Google bot to crawl all the pages of the website.
Hope this helps!