So as right now there are no effects that kick in as you spend money to pay for a card buy, it does not matter whether you will pay 1 coin then 1 potion then 2 coin then 3 coin and then 5 debt and then 4,5 and 6 coins - there is no order by which we must spend our money?
The way I understand it is all treasure you play (coins and potion) go into a "pool". Once you are done playing treasure cards, and adding treasure you got from Action cards, and coin tokens, you choose to buy a card (or more if you have more than one buy) then that amount of treasure gets subtracted from your "pool". Then any "on buy" effects happen. Taking debt happens here by my understanding. It doesn't matter if the debt is in the cost of the card, or if the debt is there as a result of the event Tax. So to answer your question, you "spend" everything you need to to buy a card at the same time. The order does not matter because you already have all the coins and potions before you buy cards. (I'm using the term pool because I can't think of what else to call this concept) Borrow is an exception, an event that you buy that gives one coin and another buy with which to use that coin is the only example I can think of right now that gives treasure after you buy it, so that's the flaw with this example I can think of.
Welcome to the forum btw, if you need more rules help, I suggest checking the wiki here
http://wiki.dominionstrategy.com/index.php/Main_PageOr download Jeebus's rules document.