As already replied nobody knows what Apple will be doing with the next machines. What you read around all just speculations. Some are very next to reality but until machines aren't released nobody knows how really they will be.
"Unified memory" means that alla parts of the computing system such as the main processor (CPU) the graphical unit (GPU) and the Neural Engine (and others) all access to the same memory blocks. They can read and write directly to the same block of memory. For example: the CPU can read directly datas thata where written by the GPU (although they treat memory block sizes in different ways) while before the CPU has its own memory and the GPU its own again and if they wanted to share datas, those had to be swapped between the two areas, wasting time. Unified memory means faster processing of datas between different processing units. Said this, you shouldn't care about technical detail unless you're a developer. Unified memory means faster machine. Fullstop. We can discuss that the M1 chip memory is a 4,266MHz while "old style" DISCRETE video card have extremely more faster memory but is not only a matter of memory. The whole M1 chipset (SoC) works in a very efficient way if compared to the traditional chipsets. I have an M1 MacMini and coming to simulators (Trainz 2019 - M1 native - and X-Plane - Intel code) the performance are really impressive for a base machine (like the MacMini, NDR) and it's only the beginning.
For your needings which you didn't clarify well I believe an Intel machine is enough. IMHO opinion, AND PLEASE NOTE IT'S JUST MY OPINION, we are not going to see any 27" iMac 'till 2022.
