One or two arduinos (3.3v and 5v).
Have a sketch for each product. For I2C devices attach the appropriate wires (including the interrupts), load the sketch, and the device starts printing out data - so you know it actually works. Same for SPI - maybe two sketches, one to test each.
Dozens of people have had problems with the HMC5843, and that has only 4 wires. I would buy a $20-$30 board if I could be sure I could test that device so I would know if I was shipped something which is broken - does Sparkfun test things? If I knew before it was sealed, it passed an identical test connected to an identical device testing it at the limits (e.g. 400kHz I2C, viewing the ID, watching numbers change as I spin a magnet), it would make things a lot easier.
I had similar problems getting the ADXL345 up and running, but it was all software subtlety. The code examples help, but they usually just are a “hello world”.
And I would rather have $100000 spent creating this than the “free day” - Instead of getting two more HMC5843 which might be broken out of the package, I already have arduinos or would get a PIC or whatever, could download the test, hook it up as specified, and know if there is any chance of getting it working as they should have been doing exactly the same things when they were put into the bag.