100% play to play. You CANNOT unlock additional build slot or research slot without paying $5 for the first, and $20 for the 2nd. There is NO WAY to purchase these things with in-game materials. They can only be bought with money. This is the problem. There needs to be some way to buy them in game via a slow-drip currency that one can pool over time. Latinum would be the most obvious choice. Make people choose between saving their latinum to buy 2nd builder/researcher, or instant gratification.