Not OP but I am in the process of upgrading my M93P Lenovo Thinkcenter. Uses laptop ram and simple to replace hard drive and RAM. I am far from a hardware guy.

Yes to both

2x DDR4 SO-DIMM RAM (notebook) slots. No ECC though (at least on this model)

And you can have 2 storage drives. One 2.5" and one M2.NVME

Yes, almost everything is replaceable https://download.lenovo.com/pccbbs/thinkcentre_pdf/m910q_ug_...

Can even add HDMI, serial ports etc.