These are the cards that give virtual $ and or +buy in the Base Set:
Chancellor
Woodcutter
Militia
Moneylender
Market
Festival
Council Room
I wonder if it would have been possible to swap them around with later cards to make for an even simpler base game (In terms of mechanics). It would have been cool to have a base set that really emphasised Treasures as the source for $, and it seems to have cards that play into that perception (Adventurer, Mine, Bureaucrat). Also a stricter rule of "you may only buy one card" (in the base set) would have emphasised the need to transition to greening in the basic rules.
So what cards would best replace them? I think Spoils are simple enough to include in the base set.
Chancellor: A card that speeds up cycling, like Cartographer.
Woodcutter: Explorer, or a simpler version that's just "Gain a Silver in Hand" for $4 covers the money+gain niche.
Militia: Ghost Ship and Torturer are too brutal for the base set. How about a horse traders variant: "Gain a spoils in hand. Each player (including you) discards down to 3 cards in hand". Too good for $4?
Moneylender: "You may trash a copper. If you do, gain a Spoils in hand"
Market: To cover the "peddler variant": "+1 Card, +1 Action, Each player (including you) gains a copper. Put yours in your hand". Probably a $4 card.
Festival: "+2 Actions, gain a Spoils on top of your deck"
And then
Replace Council Room with Soothsayer, Replace Witch with Altar, Make Adventurer cost $5, replace Feast with Coppersmith.
There's the base set as I would have had it.
PS: There's enough room for Spoils in the base set. The base set has 7 blank cards and placeholders for the base cards.