First of all.
What sort of narrowboating are you intending?
Live aboard, all year, or all summer?
Or all summer holiday plus short breaks all year?
This influences heating choices and clothes washing options.
Is access to the entire network important to you?
This sets up maximum craft dimensions.
For how many years?
By yourself, two of you, or at times a whole group?
As a couple we found a 2 plus 2 arrangement ideal.
Have a look at lots of boats. Wilton is a good place to see a lot.
And the concentration of other brokerages nearby. This limits the amount of traipsing around the countryside in your initial quest.
You will also get a gòd idea as to who to trust your boat with, when you sell.
But doing this will give you an idea what you will get for how much, and what features you consider desirable, and a basis to rank those desirabilities.
But don't over think it, or be too dogmatic.
You are best to resign yourself to having to making comprimises here, as otherwise the time boat hunting will take too much time away from actually boating.