Note that how much gold you have attained is tracked separate of your game status. Getting 100 gold in one game, reloading or starting a new game, and getting the same 100 gold will still count as having gotten 200 gold. Do not worry about not spending the gold either, as it only tracks the accumulation of gold, not what amount you have at a given time.
The easiest way to get all three of the gold collecting achievements is to play through as normal, saving up items in your inventory. Gold will seldom be a problem and you won't have to sell much of anything unless you play a Mage with limited Mana reserves. Towards the end of the game, save your game close to a merchant of any type, then sell your entire inventory. This will easily net you 10-25,000 gold depending on how much you kept around for this. Reload your game and do the process again, until Moneybags and King Midas unlocks. Well-to-do will probably have unlocked very early in the game, from quest rewards.