One can have a different opinion from me so I am advising you something I would do if I would be at your place.

I would list down all the keywords I want to target on the website and divide them in to 2 parts Primary keywords and Secondary keywords keeping their importance ad competition in mind. I will then target the secondary keywords to sub pages and try to target primary keywords on the home page.

Also, I will create sub pages for exact match keyword (for primary keywords) to give support to the home page!

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!

I agree with Matt, there is no penalty I know that has to do anything with penalty but if you run screaming frog crawler it will ask you to limit the size of the image to 100KB but again going above is not really a penalty.

Yes if the images will be heavy the time on site will increase and if the page take more than usual to load this is where you might lose your rankings as site load time is a ranking signal now.

Hope this helps!

Mate nailed it completely!

I agree with Russ! If you have a 404 page see if you have exact page on your new website (or atleast nearer to exact) if yes, redirect! If you are in to ecommerce industry and you really don’t think that 404 page have any other alternate, in that case try redirecting it to 404 pages!

Make sure the page is customized, provide value to users and allow them to continue their journey on the website.

Hope this helps!