Hi Peter,
Due to other things vying for my time I have not yet started building the Paula 3. I have planned a few modification I intend making to the model and have made an inventory of additional items I need to purchase.
To answer your question I plan to use a:
"540 DC Motor (6-12V) NOISE ATTENUATED MFA motor"
and to power this from a 6V 5Ah Sealed lead-acid SLA battery. Both of these are available from the excellent "componentshop.co.uk".
Unfortunately, I am unable to provide any feedback on this motor/battery choice, but I may do so when the model is completed.
The Paula 3 although a fairly simple design offers a lot of scope for additional features and should be good fun to build.
Good luck if you select this as your next project.
Kind regards,
Pauli