I'm generally not a fan of the insulated cavity + drylinings solution, but that aside it's good to see people concerned anout airtightness. To comment on your question:
1. You will get a pretty good airtightness rating from the wall itself if its wet plastered with two coats.
2. The problems occur where there is a break in the wall (windows, services etc.). You cannot depend on wet plaster to provide a long term seal here, as the plaster will dry out, constrict, and leave gaps.
3 The use of specialised tapes (proclima or siga being the most common brands) around openings will give excellent results if fitted properly and at the right time (in your case that would be pretty soon).
Google the above brands for more info, the suppliers are happy to advise. Also, there were 'passivehaus' type videos on youtube a while back, some of which outlined best practice in airtightness. I dont have the link to hand but it should be easy enough to find.
Also, dont forget your roof construction, as this is where a lot of problems manifest themselves.
www.studioplustwo.com