There is no residency requirement to get a golden visa in Greece but, if you want the EU citizenship, which is what everybody wants, you'll have to live for 7 years in Greece and very few people want that.

FYI: Sylvester Stalone did an ad for the North Coast in the Egyptian Midterranean recently... but in any case, if you would like more info, would happy to talk and/or introduce to more experienced people who know about short-vactation rentals worldwide (including Greece and Cyprus).

Have the tenant talk to the neighbour and see if they can come up with a compromise-eg if they are smoking can they (the person smoking) keep the windows and doors closed? 

In any case, my new business partner specializes in matching investors with developers internationally (Greece, UK,Egypt then UAE, Cyprus, Indonesia, Georgia then  Portugal, Oman, Spain, Montenegro, Qatar and, Thailand Saudi Arabia).

I would go to Greece to get my golden visa (although I don't need one, as I am also a Greek citizen).

I'm told to just get approval from neighbours and management (with building minutes as evidence) and build it quietly.

The catch for me is that I live in the neighbouring city about 45 minutes away so I want to make use I know what I'm getting into.

If you don't have a problem with it (your lease says otherwise), there is nothing in the condo bylaws about it and it is legal, then too bad for the neighbour. 

I left Greece in 2007, just before the bottom fell out of the Greek economy, came to the USA and married a Russian/Ukrainian immigrant in 2012.

I just did it myself for a place in Greece this summer for a family vacation saving almost 20% by booking direct.