Even with prep, Doom has been foiled before. He took over the earth with prep, freed Magneto for fun and dared him to take the earth back. Magneto took it back. Doom with prep created Terrax, who turned against him and locked up his armor. Doom with prep attacked Thing, who beat the hell out of him. Doom with prep AND a magic pact with demons still got owned by the F4 in the end.
But this depends on your definition of non-cosmic. Does that mean anyone below Silver Surfer?
Maybe Magneto. But I'm inclined to steer away from mutants for this fight. Mutant power turn-offers are too ubiquitous. You'd need people who have either versatile or mysterious powers.
I'd say:
High Evolutionary
Juggernaut (Doom couldn't find a way to counter Cytorrak without weeks and weeks of prep time. And even then...)
Maaaaaybe Apocalypse
Higher order Externals (like Gideon)
Sentry (if he counts) - he's Galactus level
Thor (if he counts) - waaaay too many powers and way too dangerous
Higher order Eternals (like Ikaris and Sersi)
Ghost Rider
Cloak/Dagger
Bedlam