So I think there are a couple of ways to do what you are looking to achieve. Let’s start with your first item.
Sort computers by needed patches
I’m not sure you would necessarily be sorting the computers, per se, but you could go into Sites --> External Sites --> Patches for Windows --> Fixlets and Tasks and then in the right-hand pane where the Fixlets are displaying, sort by Applicable Computers. This will give you the top patches that need attention based on the quantity of applicable endpoints for that patch.
If, however, you are looking for the machines that need the most patches applied to them, we have a number of options here, let me know if this is what you are looking for.
In regards to your second question, “which patches need to be applied per computer” - you can just click on the computer itself and in the pane the display below the computers, you should see Relevant Fixlets and Tasks, breakout that out by Fixlets Only and then by Site (i.e. Patches for Windows) you can even drill down by Source Severity (Critical, Moderate, etc.) once there.
There is also a really good report that was created some time ago that most BIgFixers get a TON of value out of which also may help solve what you are looking for. I have linked it below:
I’ll keep an eye out for your reply and see if I can assist further after you review those items.