Generally, if you're going to be buying cards with a potion in the cost, you will buy a potion right away on a 3/4 split. If you wait, then you won't have the potion shuffled into your deck early enough to get the potion-cost cards into your deck quickly.
Deciding whether to buy the potion-cost cards or not really depends on the setup. There are some situations in which a single card with a potion cost is worth purchasing (e.g., Golem in setups with poor +action, Vineyard in action-heavy setups). In other cases, it's better to ignore the potion-cost cards--if there's a Possession and Embargo on the board, lots of players will choose to embargo the Possession instead of dealing with that card.
Finally, whether to get multiple potions also depends on the setup. Setups with +buy and cheap potion cards like Vineyard or Transmute make it less painful if you draw multiple potions in one hand. If you have cards that are fueled by potions, like Alchemists, it may be worthwhile to have an extra potion in your deck.
Although it seems really complicated initially, and the strategy can end up pretty involved, my experience was that it was rather straightforward to actually play with Alchemy once I had a good grasp of the base game. I certainly made lots of bad strategic choices initially, but I was not really confused by the dual economy.
If you've played with Black Market, it's a similar kind of calculation to playing with potions: you need to have that card in your hand in order to get access to other cards (potion cost cards or the BM deck), and you decide how many to purchase based on your needs at the time and your current deck composition.