First, based on all the information out there if they default 100% the EU is going to basically kick them out. Especially, after the Greek PM just played Merkel and Sarkozy. They aren't happy with that. Thus, the Drachma.
No such thing as an import driven economy.
They can export cheap wages all they want, just not to the EU if they aren't part of it. The EU will throw up substantial trade barriers.
Of course, they won't have cheap wages if they stay in the EU and keep the Euro.
Also, no need to redue the entire tax system. Most of it would be fine, as most of it will be percentages or if it was a flat fee in Euros the currency market would set the Drachma amount. The import regulations wouldn't be a big issue, as I'm sure they already have current ones for non-EU members, now they would just add EU members to the list.
The black market is irrelevant to all of the above, as it exists and will continue to exist no matter if Greece stays in the EU/keeps the Euro or leaves the EU/100% default/Drachma. It's existence is independent of the decision. Not that it isn't a problem, but the problem exists either way.