I try to avoid using themes. Themes are presented as “do everything” with one theme solutions. I find that themes are usually not a lot of help, create performance problems and compatibility problems. So I will get started with the standard Luma theme.
I look for reputable Magento extension vendors first. Even though these extensions are usually more expensive, I expect these extensions to be better supported in future M2 versions, therefore have less issues with new M2 versions. For any given installation, I try to use extensions from the same vendor, if available. Again, expecting that extensions from the same vendor will have less compatibility errors.
I will argue that the cost of upgrading an M2 (M2.x to M2.x+1) installation is in the neighborhood of 20 hours (as an example, I’m using 20 hours). The cost of upgrading with incompatible extensions could easily double or more. This is a main concern to me.