Thank you, all, for so much advice. I've been mulling things over and doing some research, and now have even more questions (and ramblings).
I'm a little nervous of getting a 5 x 7, because I can find very little sheet film to fit it online, at least. (FP4 and HP5 at Silverprint is pretty much all I can find.) Is there a secret film source that I'm missing? I could get a 5 x 4 back for one - how rare are they? Is it possible to adapt 5x7 darkslides for 5x4 film, for the times when I want to play with a different emulsion, if I can't get the right back?
I am (amazing coincidence) going to be in Manchester this weekend, and could potentially nab a couple of hours on Friday to look at
this, if the seller's fairly central, but I'm not sure that I can properly tell a 5 x 7 from a half plate camera. So that's probably not an option.
If I were to go with a 5 x 4 (and that's seeming more likely at the moment, because I can find very little in the way of 5 x 7) I have these options;
I emailed MXV, and they have a Linhof Kardan for £99 - but no suitable and cheap lenses at present. But I could find a lens elsewhere. Is the camera likely to come with a lens board (I will email and ask), and if not, how hard are they to get? Am I best off looking for something that will let me interchange backs easily? Because it might be good to use roll film sometimes.
I think I do want something with a bit of movement - the capacity for silliness there appeals, apart from anything else. So I *think* I've narrowed things down to wanting a monorail camera. And I want a 150mm lens. I learned to photograph using a 50mm on 35mm, so starting this with the equivalent seems right.
I'll keep an eye on ebay as well, and see if something perfect-looking appears there. And I can always find the camera now (it's meant to be my Christmas present) and save for a lens. (Although how sad to have the camera and not be able to use it....)
Thank you all for your help! Sorry about all these quite clueless questions.