After talking to several other Velocity owners, I decided to go with the Odyssey PC925 battery. I also bought the mounting bracket for it as it fit well with the battery tray I had. The PC925 fits nicely in the nose and its terminals are in a good position.
I decided to take the time to clean up the interior of the nose a bit before going much further. I removed all electrics and RG stuff from the nose and painted it with a grey brush-on paint. Bit of a pain, but worth it as it look much more tidy. I then started to remount the RG system, battery, pitot heat relay, oil cooler back into the nose. Those with a keen eye may notice two different pressure sensors on the RG pump. My high-pressure switch never worked properly, so I ordered a replacement from the factory – looks like they’ve moved to a new design.