groups of cells
A normal light microscope. 4x-20x magnification
Lo resolution: Light microscope with 40x magnification.
High resolution: Confocal microscope with 40-100x magnification.
Low resolution: Confocal microscope with >=60x magnification.
Note: some organelles are easier to visualize while others are difficult. With confocal you can manage to see almost all of them with low resolution i.e you will be able to detect their location and distribution but wont be able view their fine structure.
To view the structure of organelles you would definitely need an Electron Microscope.
Is there a difference between digital and non-digital microscopes for that purpose?
If you are not too much into the optics of it then I guess it wont matter. I prefer if it has an analog mode along with a digital mode. Image acquisition is anyways digital.
Popular models: Leica, Zeiss and Olympus are popular brands. You select the model as per your requirements.