I wished a round score of men—in case of natives, buccaneers, or the odious French—and I had the worry of the deuce itself to find so much as half a dozen, till the most remarkable stroke of fortune brought me the very man that I required.
The next few sentences describing Silver's bank account, wife and inn confirm that the squire is using "substance" to mean "wealth" in the example sentence. But Stevenson could've intended some irony connected to the more common definition of "substance" ("the real physical matter of which a person or thing consists"), since the readers will discover that Silver is never really what he appears to be.
I forgot to tell you that Silver is a man of substance
While I was still in this delightful dream, we came suddenly in front of a large inn and met Squire Trelawney, all dressed out like a sea-officer, in stout blue cloth, coming out of the door with a smile on his face and a capital imitation of a sailor's walk.
like or suggestive of a sheep in docility or stupidity or meekness or timidity
Another definition of "sheepish" is "showing a sense of shame"--this also fits the example sentence. While the "docile" definition is a more obvious description of the way Morgan obeys Silver's command to come forward, the "shameful" definition suggests that Morgan knew that the man he was drinking with was the same pirate who had fought with the captain.
The man whom he called Morgan—an old, grey-haired, mahogany-faced sailor—came forward pretty sheepishly, rolling his quid.
a playing card in the suit that has been declared trumps
The informal definition of "trump" as "one who is reliable or admirable" is a better fit for the example sentence. "Trump" as a verb means to "get the better of" and while that definition is not intended by the squire, Stevenson's use of the word could hint that the squire has been misled by Silver's two-faced nature.
not affected by a chemical substance (especially alcohol)
Sometimes he fell and cut himself; sometimes he lay all day long in his little bunk at one side of the companion; sometimes for a day or two he would be almost sober and attend to his work at least passably.
The use of the word "wily" contradicts the last part of the sentence's description of Israel Hands. Jim could've been using the word to simply mean "skillful", which goes with his other adjectives of "careful" and "experienced"; but as a first-person narrator, Jim already knows how the treasure hunt and all its participants turn out, so his inclusion of "wily" could be hinting at the deceptions.
And the coxswain, Israel Hands, was a careful, wily, old, experienced seaman who could be trusted at a pinch with almost anything.
destroying someone's (or some group's) honesty or loyalty; undermining moral integrity
By a "gentleman of fortune" they plainly meant neither more nor less than a common pirate, and the little scene that I had overheard was the last act in the corruption of one of the honest hands—perhaps of the last one left aboard.
He did not know, to be sure, that I had overheard his council from the apple barrel, and yet I had by this time taken such a horror of his cruelty, duplicity, and power that I could scarce conceal a shudder when he laid his hand upon my arm.