In particular,

What class is the easiest to play? Which one is most fun?

Do you get HP from leaving dungeons after killing enemies?

How many cycles should I set DOSBox to? I want my game speed comparable to what it would be on period hardware. (That is, hardware that was common around the time the game was released.) (I'd be using the Ultima Collection version, which does not come configured for DOSBox.)

Are spells really "dungeon-only" as the reference card states?

Probably cleric or wizard, for easy navigation and exit of dungeons. Although you aren't actually required to enter any dungeons in this game.

HP can only be bought from kings.

The default 3000 cycles is fine, but you will absolutely need this patch. http://exodus.voyd.net/ultima2.html

Yup, no spells on the overworld, but you can gain some firepower by way of your ship's cannons.

It can be really difficult to get enough food early on, so I always recommend immediately going to the town near Lord British's castle and robbing the local restaurant. Just walk up to the counter and 'steal food' - any class can do this, but agility helps. Leave town when the guards start shuffling your way. You also won't consume food while aboard your ship.