Another thing to consider in addition to what you say in your post is that lap harps are far more difficult to learn on than floor standing harps.
And yes, you'll definitely want more strings even for Celtic music.
Consider renting to begin with, it's common in the harp world and a good way to figure out what you like.
There is no way I can run to a floor-stander although I have seen a slightly larger, 29-string instrument for not too much more money. If... repeat if I become so serious that I want to play something larger then I will consider renting. Purchase is a gamble I'm willing to take at this moment in time.
I pop in here from time to time as the harp is an instrument I am drawn to. It does seem an expensive choice though and not sure i can justify the expense. Unlike flute or clarinet where you can usually get a beginner's one second hand for not too big an outlay the harp seems different.
I will watch with interest Saxwarbler when you report back what you bought and how you are finding learning it. The sort of music you mention is similar to what I would like. Are you thinking of lessons.
I have to say, before I had even seen the Derwent ad, the harp was lurking at the back of my mind. Never considered it until I found myself chatting to a (frankly gorgeous) bloke who was helping one of my youth groups with a garden project. We were talking about playing instruments and he said he felt daft saying it and not to laugh at him, but he'd bought himself a table harp and was loving playing it because, he said, "even when you make a mistake you can't make a bad sound on it". Well of course, I was intrigued and I wouldn't have dreamt of laughing.
Like you, I had also thought they were too expensive until I realised that smaller harps can be had for a lot less. Ordinarily I wouldn't be able to justify the expense of even the small Derwent but as I said, I have a little windfall on the way so I'm going to treat myself.
I don't plan to have lessons. I already have piano and - once Covid regulations allow - singing lessons and I'm doing really well with both. One or the other would have to go and I'm not prepared to give up either. I've managed to download a copy of Sylvia Woods' Teach Yourself to Play the Folk Harp for starters.