Can't help with costs in the Highlands but for a building of the floor area you're looking at there's no reason why you shouldn't build it yourself if you choose to, especially in timber framing which is straightforward hammer and nail or bolt and nut technology. There are plenty of good guides to timber framing if you search on the net; you could also try a search for TRADA, the Timber Research and Development Association as it used to be known, who have an excellent bookshop service for technical books on the subject; most are aimed at professional users but worth a look.