Courtier's power level seems to be pretty polarizing. Perhaps it would help to know how likely it is for a kingdom to have at least one card with 2+ or 3+ types? I have done some counting, and this is what I came up with:
In all of Dominion (post-second editions), there are:
180 cards with only one type.
73 cards with two types (counting Catapult and Tournament).
10 cards with three or more types (counting Castles).
In kingdoms with Shelters, everybody starts with 3 two-type cards.
In kingdoms with Inheritance, there is (almost certainly) an opportunity to get two-type cards, and might be an opportunity to get three-type cards.
Given that Courtier is in the kingdom, there is an ~97% chance that a random kingdom has at least one card with at least two types.
Given that Courtier is in the kingdom, the expected number of kingdom cards with at least two types is ~2.86.
Given that Courtier is in the kingdom, there is an ~30% chance that a random kingdom has at least one card with at least three types.
Given that Courtier is in the kingdom, the expected number of kingdom cards with at least three types is ~0.34.
Of course, not all kingdoms with a two-type card are one in which it is trivial or wise to try to pair it with Courtier. However, there are so many two-type cards - and they tend to be powerful, that it will be very likely for engines to have at least some of them. Many two-type cards are treasures or non-drawing terminal actions, which makes them convenient to hold onto before playing Courtier.
My thoughts on Courtier:
A $5 card that provided $3 and +buy would compare favorably with Wine Merchant.
A $5 card that provided $3 and a Gold gain would compare very favorably with Explorer.
On most boards, Courtier is better than either of those cards, because it can trivially be either, or one of the other four combination of two options. Occasionally, in the presence of cards with three or more types, it can be great (non-terminal $3 and buy is the safe combination, but sometimes the Gold gain is really valuable). Even when it misses, it isn't horrible. A terminal Gold or Gold-gainer is sometimes good enough, especially early in the game (when Courtier is more likely to miss).
Courtier is primarily a non-attack payload card, which tend to be unpopular, but it is one of the strongest in that category. I think it might be around the power level of Merchant Guild, but time will tell.