Questions on replacing a stack

I'm having some issues with my current switch stack and I'd like some recommendations on what to do. I've searched around for a bit but I still have some questions.

Current environment: 450 employees nationwide. 150 locally. Datacenter hosted on site consisting of 5 VMWare hosts, 1 Nimble SAN, Cisco 2901 router. I have a single switch stack consisting of 4 Dell Powerconnect 6248P, 2 Dell Powerconnect 6248, and a Dell Powerconnect 6224; all of which need to be replaced.

So Nimble specifically mentions that they want buffer memory of 512k per port. This is particularly frustrating since nobody seems to advertise that number even in their spec sheets. I've found some options with a shared buffer size that is more than what Nimble suggests but averages out significantly lower than 512k per port. With only 1 SAN and 5 virtual hosts on this switch, would that cause me any bottlenecks?

Right now, all 7 of my switches are stacked. I think what I want to do going forward is keep 2 switches unstacked and redundant with each other to just service the virtual environment. Then just stack the rest and run some SFP+ between the stack and the server switches. Is that the right idea or am I just creating unnecessary work for myself?

Any hardware recommendations would be appreciated as well. I'd like to keep it all in the same manufacturer family but the big name switches are just too expensive to just buy 7 or 8 of them just for simplicity sake when a lot of their muscle would go unused for normal user switching needs.

Thanks!