I can't stop thinking about the Amazing Molly Maxwell.
She can produce campaign reward cards from deep in your deck - they are often powerful and have unusual traits. Many do use the ally slot, but this just means you can spend Molly's penultimate point of sanity to find them, and then whisk her away into your discard to play them.
The Chthonian Stone (Cursed) is another asset we'd like to run one-of but still find consistently — it's * unique. It might have particular synergy with Dexter too if he finds himself scrabbling late in the scenario for new assets to put into play efficiently.
If you want to Versatile in Scavenging and build Dexter around that card, you could make sure Scavenging is your only Talent. (Note: This is probably a very bad idea)
Assuming Dexter has Rogue 0 - 2 deckbuilding:
One copy of Liquid Courage (Illicit) might be interesting — you don't need horror healing until you do. Although you are spending sanity to find it, and this assumes your deck isn't full of Illicit cards.
On the other hand, if you do go full-Illicit, Fence is a Connection. I'm skeptical that this is the way to go, but if you do run Fence, you want to find it consistently.
Well Connected (Condition) seems strong with Renfield and Rogue money cards of choice. Perhaps as a one-of? It has 'Limit 1 per investigator' and amassing a fortune takes a few turns.
I'm not an enormous fan of The Skeleton Key (Relic. Cursed) but Exceptional cards are more appealing when you have a consistent way to draw them. And, if my understanding of the rules is correct, when Dexter finds himself unable to spare the actions to go and retrieve the Key he can discard it remotely to fuel his ability. And then Scavenge it back to his hand.