Hi Popz
Great that you are in touch with folk locally who can give you good advice (and bees for that matter!). I'm really pleased that you are already aware of the issues of the movement of stock which isn't well adapted for the location, and the possible introduction of Varroa to Varroa-free areas. Neither of these issues seem important when you are sitting somewhere in Middle England, but things are different in the more remote parts of the UK. The coastal areas of Wester Ross are also mentioned as Varroa-free. A new beekeeper who didn't have local contacts at the time introduced bees there from SW England last year and - when they died out overwinter - was about to do it again this spring. I don't blame folk for doing this if they don't know any better, but the bee trader involved had no hesitation in trying to make the sale then teasing about his sales to Scotland later. Anyone who criticises this trader on here gets stick from folk who are grateful for his ability to supply bees at a time when they can be hard to come by, but I'll continue to speak up.
Anyway, welcome to the craft and welcome to the forum.
Top bar hives? Sure, you will find beekeepers that think that they are unsuitable but then you will find beekeepers that think everyone should be using only wood, only polystyrene, only Langstroths, only Buckfast (actually I have some Buckfast in my apiary and I quite like them!). In truth, TBHs might be educational and interesting and *might* have additional advantages such as a freedom from commercial foundation (but you can go foundation-free in a standard box too) and permitting the bees to draw the comb size they want (tried that myself - and I'm not convinced that it did anything useful).
It is probably fair to say that managing your bees will be easier in a standard box, and with Amm stocks and the gentle moist climate of Mull and its flora, single National boxes would probably do the job well. Bees can be reluctant to work sideways especially with limited income, and like to work upwards where the environment is warmer above the broodnest. All this means that your TBHs might make a lot less honey than similar bees in Nationals, but give it a go by all means and let us know how you get on.
all the best
Gavin