The "40 card hand" problem is easily solved with one card: Apprentice. If you start the turn by drawing four Apprentices and a Colony, then trash a Colony to draw six more Apprentices and five more Colonies, with four more Colonies coming up in the deck, by the time you are done trashing Colonies with Apprentice you'll have a whopping 95 cards in hand.
EDIT: OK, I've tried to combine the various insights in the thread so far into a solution that:
1. incorporates the need to draw all the cards you're playing (along with +actions as necessary);
2. plays by normal kingdom-size and player limits;
3. doesn't have 3+ piles exhausted at the start of your turn; and
4. doesn't use Watchtower to trash Silvers as you gain them, as I understood the point of the puzzle is to amass as many Silvers in a single buy and buy phase as possible.
The kingdom consists of:
1. Apprentice
2. Tunnel
3. Haggler
4. Horn of Plenty
5. Ill-Gotten Gains
6. Loan
7. Hoard
8. Cache
9. Embargo
10. Black Market (with King's Court, Trader, Venture, Young Witch)
11. Shanty Town (Young Witch bane)
Colonies and Platinums are also present. Before you start your turn, there are 30 Embargo tokens on the Colony pile (from 10 Embargoes having previously been KC'd). The Embargo and Loan piles are exhausted. There are at least three players present (and so 12 copies of each victory card).
Draw Apprentice x4, Colony x1. Trash the first Colony and draw three more Apprentices, another Colony, a Province, and four Platinums (among other things). Trash the Platinums, Province, and Colony and you'll have a hand of 57 cards (5 starting hand + 66 drawn from trashing - 14 of those cards being Apprentices, Colonies, Province, and Platinums).
Those 57 cards are:
- Shanty Town x9
- Haggler x9
- Horn of Plenty x9
- Ill-Gotten Gains x9
- Loan x10
- Hoard x9
- Venture x1
- Trader x1
Your draw pile consists solely of 9 Tunnels.
Play the 9 Shanty Towns to get +9 Actions, then play the 9 Hagglers.
Now, in your buy phase:
- Play a Loan, discarding all the Tunnels and Tradering the gained Golds. {+9 Silver}
- Play another Loan, triggering a reshuffle with Tunnels on top and discarding them all again. {+9 Silver} The Loan then hits and discards a Silver; there are now 8 Silvers left in your draw pile.
- Play the last 8 Loans, discarding those 8 Silvers.
- Now play your Venture, triggering a reshuffle and plowing through the Tunnels one last time. {+9 Silver}
- Play the 9 IGGs, Tradering the gained Coppers. {+9 Silver}
- Play the 9 HoPs, gaining Caches and Tradering the double Copper gains from each. {+18 Silver}
- Play the 9 Hoards.
Now, buy a Colony.
- Gain 9 Golds from your Hoards, Tradering each one. {+9 Silver}
- Gain 9 Silvers via Haggler. {+9 Silver}
- Gain 30 Curses from the Embargo tokens, Tradering each one. {+30 Silver}
- Trader the Colony when you gain it. {+1 Silver}
That's a grand total of 103 Silvers, all sitting in your deck.
Unfortunately, there didn't seem to be room in the kingdom to make your actual "purchase card" something with a cute on-buy effect, like Farmland or Noble Brigand. As that effect produced less extra Silvers than the other kingdom cards, and wasn't essential for draw/actions, I sadly cut it.