Normally, I just roll for Comeliness but, if you want to use the formula, feel
free. It is almost certain that a PC's Comeliness will not make the slightest
bit of difference in the actual game.

Full HP for the first level; roll dice for second and third level.

Do whatever you wish for gold. Your rule is fine.

A 3-hour, one-off game is very short. You are welcome to create a full PC but
only the most important details, such as hit points, weapon, spells, class, race
and name, are actually likely to come up during the game. When time is short,
it is very hard for both the player and the GM to remember and to use details
about a PC.