Obviously, what you do depends on the Kingdom, but to really simplify things, I teach/tell players to always try to get your money's worth. Got $6, get a gold. Got $8, spend all $8. A Province or 2 early may not hurt you, but may pressure your opponent.
Buy the cards that give you extra actions, actions and cards, actions and money, extra cards/money. Get fall-through cards (+ action) if they help you. Avoid attack cards, unless they benefit your hand when played. Trashers can be good/necessary.
When in doubt, buy money, limit action cars.
All of this is wrong.
It all depends on the board, my friend. Gold is usually something you want to avoid/have only one of. Money clogs up your deck in Engines, and isn't something you need if you can buy a Province with all your deck already sans the money. Gold should almost always be bought in a Big Money deck or something where it's necessary and perhaps a few Colony games. It all depends on the board for sure.
Your money's worth idea... It's not at all correct. Think of it this way. There is a kingdom with Scout ($4), Chapel ($2), and Silver ($3). Your starting hand is 4/3. Do you open Scout Silver? Of course not. Chapel is worth the $4 (and Scout is certainly not worth $4) and Silver helps you to get to better cards. Money is just a way to get to cards you want. If you have more money than you're using to spend on cards, you're wasting time you could be spending winning. I know it's an extreme example, but if this game has taught me anything, it's that there is NO SET RULES TO PLAY. There are general guidelines, but there's always an exception. Is there a board where you DON'T want chapel? Sure. It's very rare, but it's there.
Getting Provinces early is okay for new players, but you almost certainly won't be doing that with a good +buy card on the board in an Engine setting. As said before, it's all on what the board is.
When in doubt, buy action cards that aren't terminal. Though this isn't a rule anyone should follow either. What I would encourage is to let people do their own thing, fail, and learn from it. Like me. I don't feel like I'm learning when I lose, but really, I do.
And Attack cards slow your opponent. Sea Hag doesn't help you at all, but it's just so strong anyways. If it helps you, awesome! But if it doesn't, it needs to be a strong attack. Which is why Saboteur fails, but Sea Hag does not.
I don't know really, but I feel like what you're saying isn't right from what I've seen in high level play.