Hey, Di Valenti
It sounds like you've got a pretty good set up there! In response to your questions:
1: Since the G751 is pretty well equipped and you've got a good mouse and a good bag to carry your beastie around in, I'd say that's just about all you need. If you don't already, make sure you get a good pad for your mouse though - that can make a world of a difference when gaming. In terms of software, I can't say that I use any specific gaming software other than Steam and Origin to download and play games - but if you're interested in overclocking and tinkering with your system I'm sure that there will be others on here with suggestions.
2: I upgraded to Windows 10 last week and it went very smoothly indeed; all you need to do is download the ATK package and the ROG Macrokey drivers from the Windows 10 section of the download area. The rest of the apps that you need can be found in the Windows 8.1 section and work fine with Windows 10.
Once you've upgraded, it should effectively be like a fresh install of Windows 10 if all goes smoothly, so all you should really need to do is to removethe Windows.old folder (which will be your 'old' 8.1 install), update those drivers and you should be done. If you really want a completely 'clean' install, you could always use the reset function in Windows 10 and choose not to keep your old files and software and it will then restore everything back to scratch but keep your drivers intact. Then you could use that external hard drive to create a system image using the Windows 7 backup or use Macrium backup and restore it using that image if anything goes wrong in future.
Basically, there's lots of options! Also bear in mind that once you upgrade to Windows 10, you can get rid of that 15Gb recovery partition on your drive, which will be a pointless waste of space.
Asus ROG G751JT-T7115H
17.3" IPS, Intel i7-4720HQ, 16Gb RAM, NVidia GeForce GTX970m, 256Gb M.2 SSD + 1Tb SSHD