I prefer to use
Hass avocados. You can tell they’re ripe when the skins are no longer green but black, and if you press them with your thumb, it’ll leave an imprint. I advise if you’re shopping in Paris, where touching produce is frowned upon, that you shop by color, rather than texture. (Can you blame them? Who wants to buy dented avocados?) Then use the touch test in the safety of your home.
Fresh chilis are certainly a matter of preference. I had some slender Thai chilis on hand, and added a whole one to my guacamole, which got hotter and hotter as it sat. I generally prefer Jalapeños, if you can find them. Use however much you are comfortable with. Start with a small amount, then taste and add more. Remember that it’s easy to add more chopped chiles than to try to take them out. If you like chopped tomatoes, you can add those at the end.