What type of boat are you looking for?
I recommend visiting a boat brokerage who will have many boats available for sale. There are many like Whilton Marina who will let you have a tour of the boats available and this will hopefully give you a better idea as to what's available within your budget.
Also, sites like Apollo Duck (arguably equivalent to the auto trader of boats) will also give you a good feel for what's available.
https://narrowboats.apolloduck.co.uk/
Also do research on what kind of layout you would like, reverse layout narrowboats are increasingly popular but may not be for everyone for example. Then you have a choice of sterns to choose from, e.g. traditional, semi-traditional and "cruiser". A lot of it is horses for courses but definitely research on this if you have not already.