Almost every narrowboat on the market has 6mm hull sides, apart from Springers which are thinner, so this isn't really a thing to worry about.
It's an ex hire boat with multiple cabins. Fine if you have a family to house, but if you are a single or couple you will probably want to take some of the bunks out. Which then leads to the question whether having several small 'rooms' is what you want, or whether you would be better with a boat with fewer larger spaces.
Forget about compost toilets. CRT have banned the disposal of 'compost' (in reality just bagged poo and paper) in their refuse bins, and if you have nowhere on land to store it while it decomposes, it's just a headache. The boat has two pumpout toilets. There might be one or two waste tanks which they empty into. Remove one toilet and the second tank if there are two, and keep the other one. The saving in cost by not replacing the toilet will cover your pumpout charges for quite a while.
In queries like this I normally suggest the buyer move the boat themselves rather than paying for road transportation - the saving in lorry and cranage costs could allow you to take a week or two's unpaid leave to make the move. But in this case with a length of 66ft I think the boat may be too long for the Rufford Branch, and possibly also too deep a draft for the Ribble Link.
That in turn leads me to wonder whether moving a boat this size to the Lancaster canal would materially devalue it. Not an issue if you intend to keep it long term, but could be if you need to sell.