HI there, I've actually built my own hitbox, so I may be able to help you with this.
The Sanwa OBSF-24's are what you need and the 30mm for your jump button. What you could do with is a 'daisy chain ground' wire, which you can attach to one button and then to the next etc, so that all your buttons are grounded.
You only need a small wire to wire up your buttons. I paid about £2.50 for a 10m reel I think. It is basically just a small reel of wire used for circuit boards, I went for a red wire, although you can use any colour.
You don't need any solder or switches. What you do need is 2.8mm wire crimps. Basically what you do is strip the end of the wire you want to use and then place it in the crimp. Then use a pair of pliers or equivalent to secure the wire into the crimp. When it is secure you can then just 'plug' the end onto the other terminal on your buttons (there are 2 terminals, one for ground, one for your other wire). It's very easy and you should have no problem.
When doing your buttons, I would concentrate on doing your Home, Start and select first. Once these are wired up, group those together and then secure them with a piece of coloured tape. For the top row of buttons again, once you wire all 4 up, group them together and put coloured tape around them. Then do the same for the bottom ones. Basically this just make everything tidy and if there is a problem with a certain button, you will be able to access it easily.
If you are just using it on PS3 and PC then yes go for the Cthulu. But if you want the SOCD cleaning, I believe you should go for the MC Cthulu (This is the one I have). I have the firmware from Toodles for this, so if you need the firmware, drop us a PM and I'll sort out sending it to you
.
I recommend though getting a Nuutrik USB mount as Esku says. This will make it a lot easier for you because you only have to drill one hole to put the mount in. Also you can attach and remove the cable from the back so that it's easier to carry and you wont have to worry about damaging the wire!
If you need any help or you have questions, please do not hesitate to ask me!
PS, if you want to see my Hitbox™ please check out my profile as I have posted a picture on there