I am currently doing this same modification, I am pretty new at this and could really use your advice. I am at the point where I am trying to figure out the endstops, I have been at it most the day. the wiring that you have going to the board are you using the outer pin config or are you using the 2 outside pin config, I think its ground and signal? oh and thanks for the FW I tried to figure out how to altar it and wow thats tough. I hope to hear back from you soon as I am going to load your FW and start going through the task of figuring out which pins will work.

Typically you want "NO" contacts. (as opposed to "NC).  (just to answer the switch wiring)

Red to 12v
Yellow to GND
Green to PWM B12

Here are the pins on the board (disregard the wire colors, this one isn't mine)

Have a look at the Azteeg X1 (sanguinololu equivalent) and the X3 (RAMPS equivalent).  The X3 has tons of expansion possibilities.  You can 4-5 controlled fans or LEDs in addition to other expansion areas.  It would mean making some changes to firmware, maybe adding some custom M codes. 

One thing I want to do is have the 40mm controlled fan mounted on the extruder, and a 120mm fan mounted to an acrylic side panel.  If I want to print PLA, I would add a Fan On command for the big fan in the start.gcode to cool the whole print area without needing to take off the panels.  I would also print a cover that would clip on to the inlet to seal up the case when printing ABS.

I've got a few bells and whistles that I plan on adding to my machine.

still not sure whether to throw all the electronics away and start again with RAMPS, or whether to use the SPI bus that the SD card reader uses and make a whole new breakout board for things like fans etc.

(any suggestions)

Can you post up your modified firmware for others to use in the future?

Was the endstop business mostly a matter of setting the Pins.h for Max endstops?  Since the board I'm getting supports 6, I'm hoping it's possible to enable both Max and Min endstops, while defining Max as the home position.

No, it was more of a getting the min/max right, since the ramps also supports 6 endstops. Didnt change anything in pins as far as the endstops goes, just in the configuration.h

Step 1.  Buying HW
Buying the Ramps 1.4.  I bought a complete kit on ebay with new Pololu drivers (that I didnt end up using). My kit costed me $150.

Step 2. Making and Ramps adapter for solidoodle
Making a adapter plate for fitting the RAMPS where the Sanguinololu fitted before. By merging a Sanguinololu and a RAMPS bracket i got a design that looks like this:
https://tinkercad.com/things/6HalxdlChf … odle-ramps

And when fitted, lite this:

Step 3. Buying new PSU
Buying a better PSU. Since the RAMPS specification says that it needs 12v 5 Amps to run the motors, 12v 11 Amps to run the heaters and bed, or a single 12v 16Amps input, I bought a 500 watts computer PSU.
Looking on the internet I ended up with a stripped PSU with only the 12volts left.
The PSU can deliver 2x12V@18Amps, So i used both rails and made two separat 12v connectors.

Step 4. Mapping the connections and connect the wires to Ramps
Documenting the current connections and mapping them to the RAMPS connections. This was pretty straight forward, since RepRap Wiki  has diagrams for both boards. I used the old and moved the Pololu drives from the old board to the Ramps, so I don't have to re-calibrate them. I did though add the included heat sinks to them that came with my kit. Took a ton of pictures to be sure that I knew where everything was before I started.
The wiring was ok for everything besides the endstops that had to be rewired since Ramps use pin 1 and 2, and Solidoodle org 1 and 3.
The LED an the extruder Fan also had to be changed and connected directly to the 12v 5A input.
In the picture everything is in it´s place, also added a extra wire to the fan that can be controlled with Gcode.

Step 5. Marlin FW update
Update the Marlin FW to run on the solidoodle with Ramps. This was basically to modify the original Marlin fw boardtype 33 = Ramps,  with the config done by Lawsy (Thanks for your work Lawsy). There was some changes in the endstop handling that made my Saturday evening "fun"

Step 6. Wiring every thing up, connecting power, pray and press "home" in Repetier.
After a couple of tweeks, most having to do with the Z endstop, things started working as it should ! Heaters stared heating straigh away, but the extruder extruded in reverse. Changed in the FW and I was all set. Allso configured in the fan, and a ultrapanel LCD.

Here is a video of the complete setup:

Step 7. Cleaning up the wires and making it look good

FW included

Here is the Ramps/Solidoodle modified Firmware, also set up to use the new ultrapanel with smart adapter. Remember to set board type to amega 2560 before flashing this fw.  Requires RAMPS and has nothing to do with org solidoodle.