1. Keyboard, touchpad, poor performance of the entry-level WLAN cards. The WLAN issue is easily fixed by simply upgrading the card. The other two, depending on what the problem is, are not reparable. There are some basic steps to try and fix software-caused problems with the keyboard and touchpad. But if a brand new G74 has either of these problems from the outset and the software fixes don't help, the best bet is to assume you got bad hardware, return the unit and simply get a different one.
2. You included a lot of questions there! If you buy a one-drive configuration, you need to make sure that the SATA board and cable are present if you want add another drive. They should be, but aren't always. If you buy a two-drive system, it will have all the required parts....you'd simnply be swapping out one of the hard drives for the new SSD.
As for all the other questions, those are covered in a lot of threads here, since owners make (and talk about) this upgrade all the time. If you have questions about setup after you get the machine, start a thread for that and there are many members here who are happy to help walk you through it.
I would say the minimum SSD you'd want is 80Gb, unless you use really big programs. IMO, something around 120Gb is ideal for most people who save all their user files to the 2nd drive. This offers plenty of space for Windows and your programs, without worries about the drive getting filled too close to capacity.
3. The G74 can use both SATA II and III drives.