Hi Nick,
I'll try to answer your questions to best of my knowledge, hopefully others will chime in as well.

1. Is there clear information available for a beginner to install and get this system running?

Don't know about any instructions but as long as you're using MicroCodeStudio it's really quite straight forward.

2. How many seats are allowed to be used with the latest PBP (3.1 if I’m not mistaken) and MCSPX (5.0)? If only one can it be easily moved from one system to another in case that it is needed?

I believe it's two. I've just switched computers and the license manager allowed me to activate PBP on the new computer without first deactivating it on the old.

3. Are these two software pieces enough to get me going (write code and generate .HEX files) or I need some other applications? If I need other applications what will they be, what revisions are recommended and what will be the advantage?

You will need to install MPLABX as well since that is where the assembler is. The assembler is what gets you from a compiled program to the .hex file you program into the PIC.

4. Are these software pieces fully compatible now with Windows 10? I will also purchase and dedicate a computer to this application and Windows 10 seem to be the best choice.

Well, fully compatible, I don't know but I'm running PBP3, MCSP, MPLABX on a Windows10 machine now it seems to work for the short amount of time I've done it so far.

5. I owe a couple of U2 Programmers, one Field Programmer, a couple of PICKIT 3 devices, one LabX1 and one LABX2 boards. Do I need to invest in something else or these will suffice? I understand that pending the processor I will be using I might need some specific development board but right now I just want to get things going and move to the 18F family and the newer 10Fs that I understand are now supported. As I said before I learn slowly but I forget really fast so, small steps will be best for me.

Since you have a PICKit3 you should not need to buy anything else. Windows 10 apparently is very strict about what device drivers it allows you to use so the U2 and the Field Programmer MIGHT not work. Hopefully someone else with one or both of these devices will be able to tell you for sure.

/Henrik.