I'm probably going to pick/main Engineer if that matters.
The most important thing to note is that by level 80 you'll have unlocked everything anyway, and thus be able to change around your build freely. That said gear is still an issue, but engineer is a great choice when it comes to that because they can do so much. The main thing you want to do is simply decide your "role." Guild Wars 2 doesn't have strict roles, but it is still better to build more or less for a general one. For example if you want to play support look for stuff that boosts your boon duration and healing power. If you want to place more defensive opt for toughness for the most part. And never overlook power or condition damage for your DPS.
For example I favor an aggressive build based on condition stacking, so I go with condi-damage and precision gear. Within that framework I have lots of options for the specific skills and traits I use, as well as weapon, but I stay overall within a theme. I guess that is the main takeaway; pick a theme, not a build. Within a theme/role you have lots of build options without the need to worry about carrying multiple gear sets. However if you want ultimate versatility (the engineer is the most viable class for that) look into celestial gear; you'll have to craft that yourself since it is account bound stuff, and it only appears at max level. Celestial stuff boosts every stat, so while someone playing a more focused build will have higher stats than you and be able to do things better, that is if they limit themselves to that one specific thing; but you can do everything and freely alter your build constantly to play whatever is cool at the moment or works for given content with zero concern for your gear. Don't expect to be your party's MVP with that kind of build, but you'll provide a solid "jack of all trades" foundation for parties to form around.