I wouldn't struggle to pay the ongoing costs of a steel narrowboat, and would without doubt pull it out of the water every 2-3 years for blacking. My biggest concern, and the reason I'm mulling over a new, but lower cost boat, is that I cannot guarantee there aren't any horrors with second hand. Dodgy surveyors, get out clauses in surveys, the strongly held opinion by many that surveys aren't worth the paper they're written on, careless or neglectful previous owners, and so on and so on. All of those considerations don't exist with brand new. But, I also don't have nor want to spend £150k, nor do I want a larger than 40ft boat. I've lived my whole life in small spaces, and favour minimal possessions.
@Steve_C thanks for that link, it was interesting, and worth knowing about Cunliffe boats offering brand new 35ft boats.