First, the guide mentions "composite sync output" (white wire) and it implies that it's an optional add on for this mod. What is this for and if it is essential, where can I purchase it?
It depends on if your SCART cable uses composite video or composite sync as the sync signal. Most that I've seen use composite video for sync so you should be OK leaving it out in that case.
Second, I've heard that .75 ohm resistors are the recommended value, but I've also read that 120 or other varying values are more desirable; How do I gauge the correct value?
First off, it's not ".75" ohms as that value doesn't exist; you're talking about 75 ohms. Anyway, that's the desired value since 75 ohms is the standard impedance value used in a lot of TVs and monitors.
Third, it's actually a very simple question, the guide mentions attaching the wires to the pins, how would you attach it?
Solder them to the pins, not sure why you'd need to ask that.
Fourth: It talks about a clear insulating compound and it seems it's added for further protection; where can I purchase this?
Finally, my HDTV doesn't have a SCART input, so I want to purchase a SCART -> HDMI converter; is this a good idea? I am referring to converters such as these - http://www.ebay.co.uk/itm/SCART-To-HDMI-1080p-HD-UPSCALER-Converter-Adapter-FOR-Sky-Wii-VHS-STB-DVD-HDTV-/300614927083
I have no idea what the guide's referring to, I've modded plenty of SNES minis for RGB and S-video and never needed "insulating compound". Anyway, a SCART to HDMI converter would work, but a SCART to component converter might be better if your TV has component input and
supports 240p over component.