Why limit it to Actions? I get why you'd exclude Victory cards, since they'd be auto-includes, but why not allow Treasures?
Anyway, I think you can do just about perfect (ie, it's never right to buy anything but treasure) with the Action-only rule:
For now, let's assume you don't get perfect shuffles. That is, the question is "What is the spread you would least like to see come up in a competition to buy all the provinces in solitaire on isotropic".
Cards That Are Strictly Worse Than Silver
1. Great Hall
2. Village
3. Walled Village
4. Feast
5. City (It would be an insane waste of time to empty a pile, much less 2, when Big Money can get you there as fast as it can and no one is helping)
6. University
7. Monument
8. Throne Room
Cards That Probably Help You Less Than Money
9. Scout
10. Outpost
I'm not sure which of Scout and Pearl Diver is worse. Scout has the potential to make a much bigger impact, but Pearl Diver has the issue that if you draw $2, you're going to buy it, because it adds *some* value, unlike all your other options. Hard call.
If instead you do get perfect shuffles, it's actually possible to prove your answer, though I think it's more work than I want to do. Outpost definitely gets replaced with Pearl Diver. What I don't know is whether Scout gets replaced by Workshop. I'm just not sure whether having extra silver is better than skipping four victory cards all the time.
You could certainly make this worse by replacing a few cards with Thief and Pirate Ship, which are totally useless, but I wanted to stick to the no-attacks rule.
If you allow Treasures, I think you can be absolutely 100% certain that no one should ever buy anything other than Silver, Gold, and Province by making the last 2 cards Quarry and Harem.
@DG -- there are a lot of things wrong with that spread. First, it violates the actions-only rule, but more importantly, it gives you all these buys, and has KC in it. TR is useless because you can't net cards (or money, TR->Monument is no better than 2x Silver) with it, but with KC you can, and since you have buys, chaining them might actually be a useful thing. Hamlet is also bad as it costs 2. And FISHING VILLAGE!? With KC around!? Waaaaaay too good.