If you brew espresso, a burr grinder is necessary. Burr grinders fracture the bean along its natural fracture lines, blades "cut" them, which means breaking more cells, losing the fermentation gases which means losing more aroma.
The espresso machine will then make "explode" the cells in the filter. The fermentation gases, which are very important in the aroma of your cup, end up in your coffee as foam (unlike in cartridges models, where the foam is simply obtained by mixing ambient air into the coffee...).
Another thing you look at is how many positions: if you brew espresso, the grinder must be able to grind very fine. The fineness of the grinding varies with coffee to coffee (and with day to day). Don't assume your grinder is proper for espresso. Also, it must have many positions, so that you can use it for moka, "napoletana", American coffee, or Turkish coffee.
The way the grinder "delivers" the grinded coffee is also important. If you use espresso, you might prefer a grinder which makes the coffee fall into your filter.
You should also see reviews on easiness of dismounting and inner cleaning.
Regarding positions of grinding, the best is to have a double regulation: a gross regulation and a "fine-tuning": coffee reacts to ambient humidity (you realize that only after you brew your coffee, so if you brew a litre at a time, little use).
If the grinder only has fixed positions and no fine-tuning, that's not good. Choose a grinder with continuous variation.
I would also check that the burrs are pretty standard. If the grinder uses specific burrs, that for me is a negative point.
A €20 - €30 grinder is going to disappoint you from day one. A grinder lasts for decades. Spend.
As a famous Lavazza advertisement used to say: Coffee is a pleasure: if it isn't good, what pleasure is it?
PS Avoid manual grinders unless you want to save money for the gym. Personally, I would avoid any electronics. My Gaggia MDF grinder works as new, after 20 years. I modified it to obtain continuous regulation. I also broke the plastic lever but I did not bother replacing it.