Even if you found a range of O scale buildings they may not suit your purposes for a game. A correct scale apartment block in 1:50 is going to be massive. Nearly all gaming buildings are under sized simply to allow it to fit on a table. You also need to consider not just how it looks but the practicalities of gaming. Are the building to have figures enter them? Or are they just the backdrop/line of sight blockers to action on the street.
I would say look towards mdf to cover a large playing area quickly with modest cost. Mdf suits modern buildings and Stalinist apartment blocks because they tend to be very plain and boxy anyway.
Sound advice, thanks.
I have been looking at the dimensions of some O Gauge buildings on line, for example I found a New York type 6 floor apartment block, which measured 18" in height and had a foot print of 6" squared, that would equate in to 46cm high with a foot print of 15.2cm. Most apartments in Germany tend to be circa 3 floors/stories high (although there are the usual 'Tower Blocks' also), so these dimensions would not appear to be on the large side. I'm planning on building some myself too for the love of the hobby and time and cost is not an issue, it's more about the involvement. I have lots of Foamcore at home and have ordered some HEKI DUR walls, so lets see how the first plan comes out!
The buildings will be primarily for LoS and backdrop/atmosphere, I had initially thought to make them all accessible and build the interiors, but that would be a huge undertaking, so I'm planning on building separate interiors, perhaps 3 different types for variety, so when a character enters a building we can switch to one of the interior scenes.
Thanks to everyone who's contributed to the thread, I always love to read and hear about others advice and experiences