Darren, A very interesting endeavour which you are embarking upon and I commend you. Firstly, it always best to contact the curator in charge and ask regarding photography. Many museums will charge you for their own photographs as they won't allow you to take your own photographs. Having said that, if the photos are purely for research then some museums will allow you to take a few photos here and there. I'm currently embarking upon research on the iconography of late Anglo-Saxon pennies and have arrangements with the Fitzwilliam and the National Museum in Copenhagen to view the collections and take my own photographs. Even so, the majority of photographs I shall be using are from private collections. Whilst it's great to be able to see a museum collection if you're looking for photographs of some types I would be inclined to say that there are coins that will fulfill your needs in private collections and all you'll have to do is ask. If your guide is for commercial purposes then you are more likely to face barriers. Kindest regards, Clive.