First of all, unless you are a whizz at graphic design (which you might well be in light of your business) I would get a professional to put together the design for you, otherwise it will look amateurish. You could pay for a design in the form of templates and then do the rest of the work yourself (in which case you'd make a one-off payment to the designer for the templates and then have to sort out hosting etc yourself), or go for a full service whereby you collaborate on the design and from there provide the content to your web person so that they do all the techie stuff (in which case you'd be paying a monthly fee, most likely).
It isn't difficult to construct and manage a basic website, but if you're pushed for time or not particularly interested in web stuff then it'll probably be better to get someone else to do this for you.
"Does it matter how large the site is in terms of pages?"
Nope, as long as the site is well designed in terms of navigation. Mind you, I'm not quite sure what you mean by "does it matter": do you mean "will it cost a lot more" or "will it be technically difficult" or ...?
"Do you pay the designer per page? We want to design most of it ourselves and go to them for the main advice/how to make it work."
This will vary - you could certainly go to a web designer and pay them to construct the page templates to your design and advise you on how to proceed from there. Or you could pay them to do it all for you.
"Does it cost loads to have moving images/scrolling photos etc?"
Not necessarily, but I would only use that sort of content if there was a very good reason for it (e.g. 'virtual tours' of jobs you have done). Again it depends on the precise nature of the content you want as to how much it would cost to create. There are lots of tools available that enable you to create this sort of thing yourself (montages etc).
I'm sure one of the web design mners will be along soon to give more specific advice on the various options open to you. I've got some techie knowledge but my work was more on developing e-commerce sites, content management systems and the like for big companies, so I probably won't be of much use to you!