The length is more critical than the brand or model. How did you decide on the length, 75cm? Where will you spear, the Med? If so, I wonder if a longer speargun (e.g. 90) might be more appropriate?
You don't need anything too fancy but it is usually best to avoid the very cheapest models if you plan to spearfish for years to come.
Re. Brands, I really like Omersub but I hear they are not the same since the company got bought out ( sadly, I hear that a lot these days) and have become expensive where they used to offer good value. Rob Allen for tough gear for big fish. Beuchat have some nice models e.g. Hero and Evil but many other as well. Apnea (Turkey)'s Whaler model would be a great beginners gun with advanced features like a quality sharkfin spear, dyneema wishbone and rail it should keep you happy for many years. The twin band Apnea ST is also excellent (I have one) but costs a bit more.
There are lots of good brands and models. Several very good Greek companies, many good Italian companies, ditto France. I think there are some good Spanish and Portuguese companies too but we rarely see them.
I tend to avoid Picasso gear, not entirely successfully

. Cressi had some problems with their sharkfin spears fracturing and did not replace them (in USA). Mares had some cool looking spearguns a few years ago but I think they some kind of problem.