Should I delete this character?
It’s not the level, it’s the unlocked skins and dyes and waypoints. Me, I don’t delete anything I’ve put that kind of time into, I might want it later (I’m a total packrat that needs bank inventory interventions from friends, though). Unless you really need that character slot for something else, I’d keep it in case your tastes change again.
I would never delete a level 80 char even if you needed another slot (I would buy a slot in that case). Too much time invested to just delete something you might want later.
As far as your guardian build is concerned. There are only so many builds out there that are legitimate. People have put a lot of time into researching and testing builds that actually work very well. Why try to make your own that will more than likely underperform? There is a thread in the dungeon forum about guard builds that show a bunch of different ones. I would check into that.
As for what class to suggest, you haven’t given much info to go on, but take a couple chars to level 10-20 and try em. Or you can always go to the heart of the mists with any char and try all the traits and weapons because it levels you to 80 and maxes all your stats. That way you can try some builds without having to put too much time into each char.
For all that is good in this world, do not delete a level 80. As was said, you might regret it later, and you can always use the extra space/variety. If you don’t like any of those three, you still have two slots to experiment other classes. Guardian is a pretty solid choice, so just tinker around with it and see if you find stuff to your interest.