So apparently, the Gertboard is not going to be made available as a kit anymore. I'm glad I built one kit and I still have an unbuilt kit.
But that doesn't help you. You wanted to do some soldering.
Well, if you dont mind doing something that is unrelated to the Raspberry Pi, but always wanted to build your own complete computer from a kit, then over on kickstarter, David Griffith is trying to get another run of P112 boards and kits going. He is a little over halfway funded, with 1 week to go. Check out the video:
The P112 Single Board Computer Kit is a little 8 bit retro computer based on the Z80 (actually a Z180). Many computers from the golden era of howebrew and personal computing ran a Zilog z80, such as the RadioShack TRS-80, the Osborne, the Kaypro, Sinclair ZX series, Amstrad CPC. It was also used in some musical instruments, such as my Sequential Circuits Six-trak and E-mu emulator.
There was a previous run made around 2008, and before that, probably about 1996. It is capable of running zdos, cp/m and uzi, the unix for z80. Some software available at Sourceforge.
So, head over to kickstarter and check it out. The full kit will also require a 5v power supply and a 3.5" floppy disk drive.
Disclaimer: I have made a contribution as a backer, but beside that, I have no financial interest in this project.