Hi Greg, and welcome to the madhouse!

I wasn't aware that R.L.T. still did the plans for the 'Minim Oak', have you had them a while, or bought them recently?
I too am building a 'Moke', based on the 'Australian' built version. I took all the measurements from a full sized one, and then reproduced them in 'Half Scale' for the car. The build thread is in the 'Custom' section, under 'Secret Project Reveiled'.
Looking at your pictures, you can see the simillarity to the real car, I just hope that mine looks something like when I've finished!!

I have managed to source some 6" wheels from a Suzuki quad, but they will need to be narrowed for the 'Moke', but should look like the 13" items used on the Aussie one.
Finding a Mobility Scooter shouldn't be too hard, Ebay is full of them. Personally I would go for a 4 / 8 mph one, as these have all of the Lights, Indicators, Hazzard Flashers etc, and are quite capable of moving an 18+ stone adult about for 20 odd miles, so four little-ones shouldn't be a problem.
I have also used a Mobility Scooter to power Harvey's Toylander 2, and a full description of the build is on my website, which you might find useful. Just click on the 'Globe' icon under my username at the top left of the screen, and it will take you straight there.
There is also some information on the chain drive upgrade in my 'Hi to All' thread in the 'Toylander 2' section.
Hope this has helped you, keep us informed,
Keep Busy,
Regards,
Mark.