It's all dependent on the personality of the horse and the training they have. I have a Quarter horse mare and while anyone can handle her on the ground, there's no way I'd put a beginner on her. She's completely unflappable but she assesses her rider and then proceeds to take advantage of them in every, last, single way possible. You need to know enough to establish from the start that *you* are in charge and will not be putting up with nonsense and then she's fine but a beginner will not be doing that.
I just bought a lil Morgan, one very level headed lil guy for being 4. Completely the opposite of the age/breed I was looking for but he's perfect for me. My trainer picked him out based on his training/personality and how it would mesh with mine. Took her 6mo to find a suitable horse, we literally considered hundreds.
Find a trainer you like and take some lessons. Then when you are ready, have the trainer assist you in find the perfect horse for *you* and don't buy anything the trainer has a financial stake in, if the trainer is pushing you to purchase from them, find a new trainer!