The size of the particles of coffee that are used when brewing or extracting, play a massive part in the end flavour, body, acidity and sweetness. A coffee grind that is too coarse will be result in coffee that is weak, watery, sour and / or salty. However, go too fine and the coffee will taste burnt, ashy and smokey.
Ideally, you want to grind as fine as possible to increase sweetness without over extracting.
How fine you grind depends on a number of variables that cannot be generalised but I will touch on each briefly.
Change of Batch / Freshness
Coffee freshness or differing batches will have a more subtle affect but it will still happen. Mostly this will occur to the different moisture and CO2 content from an older batch to a fresher one. A little tweaking (either finer or coarser) will resolve this and generally the change will be lesser than a change in blend.
The higher the brewing temperature, the faster the solubles in coffee are dissolved in water, thus increasing the rate of extraction. Therefore, the higher the water temperature, the coarser the grind may need to be and vice versa for a lower temperature. This is based on taste ultimately but if you decide to brew hotter, consider starting with a coarser grind than usual as well.
Your grinder is probably the most important part of your home or cafe setup. Particle size and uniformity have such a massive impact on extraction that skimping here will not do you any favours.
A decent espresso machine that can produce consistent pressure and temperature is all you need. There are many out there and some have built in grinders as well. Money spent here will save you drama and bad coffee in the future.