All I can say to help is that I paid a deposit subject to survey, engaged a surveyor of my choice, the vendor (a boat hire company) gave me the details of a nearby dry dock so I could book and coordinate them and the surveyor. I can't remember if I or the yard took the boat for survey, but I think it was me. Survey was OK apart from a load of nitpicks, so paid in full and had it blacked at the dry dock.
I think you said that you know little about boats and in that case the fact that is has an uncommon UK inland engine, marinised by goodness knows who, would cause me concerns. If you have to get professional help someone who relay knows that engine is likely to eb difficult to find, as are engine parts. Like wise, what constitutes "good stuff" in your eyes. There is a world of difference between, say, a Victron or Mastervolt inverter and a flash looking job from Ebay/Amazon.
As long as you accept that it will take you three to five times longer than you think and cost three to five times more to complete it, it is one way, and probably very rewarding. BUT if/when you come to resell it and want to avoid having to reduce the price, or pay for a Post Construction Assessment to ensure it complies with the Recreational Craft Regulations (RCR/RCD) you need to ensure you fit out to the RCR and that requires access to a load of ISOs to be sure, nad possibly proffesional fees to help you. I fear this will open up a whole lot more debate about the value of the RCD/RCR and the need for it, but it is an aspect that many self fitters seem unaware of and it is a legal requirement, even if it is not actively enforced.
Edited to add: I used to be involved with fitting out GRP hire cruisers and despite our best efforts and Gantt charts I think we never ever hit the end dates, all fit outs over ran for various reasons, and the chippies could handle and cut 8x4 ply sheets inside the boat, that will be much more difficult in a narrowboat.