Coins will change things the most I think.
Overpaying is fun, but it doesn't seem more than a cute tactic you can only pull off every once and a while. With Doctor, by the time you can spend a decent amount on it, you'll have good cards mixed in with your bad. With Butcher, overpaying automatically means you have at least $5 and those are often best spent on a $5+ card anyway.
I'm assuming you mean Herald instead of Butcher there.
Just spouting off:
Coins will probably be better for either decks that want virtual coin (e.g. scrying pool, minion) or decks that tend to stall towards the end (because you can store up coin tokens early while you're still cycling and producing $8+ normally, then use the coin tokens to help bump up those later $6-7 hands near the end, so as to finish off the pile). I'm really not sold on using them as a megaturn enabler, at least judging by the cards we already saw; you'd have to play baker 8 times per province, and that just seems too slow.
Overspending is harder to evaluate, because it depends so much on what the overspending does; while we can guess that there may be some card that you can overspend to get coin tokens, we don't really know what the other options look like. Since it's on-buy rather than on-gain, that limits the options. That said, I can't help but wonder if early game coin boosters like Baron will get better; Doctor seems like it wants to be overpaid for early if at all. At the same time, earlier is harder to afford the big spending (both in absolute cost and opportunity cost). Herald obviously likes things like KC-KC-Bridge-Bridge-Bridge, but is worse than most villages in traditional engines. And while you can buy it to set-up Treasure Map turns more easily (both to cash them in and to match up the 4 gold with a +buy), you probably don't want to play it while you have treasure maps in your deck, unless you want to risk it trashing them for you.
Journeyman seems like a solid big money card, but it can also make engines work with much less trashing. At the same time, it's much more vulnerable to ruins than it is to curses, whether you want to trash them or skip them.