I've mentioned previously that in Edgar Wright's tradition of collaborating with actors he's worked with before, he'll want to work again with Spall. Spall has appeared in many of his movies, and with his star on the rise, he's probably in need of a giant franchise to really boost his cred.
This casting goes beyond simple logic though, he's a great actor who can portray all the best (and worst) traits that Pym has to offer.
When casting someone as Wasp, I looked for great actresses who are cute, funny, and charming. Many actresses fit the bill, but none scream Janet Van Dyne to me then Mila Kunis. She's a strong actresses who's hilarious to boot; what's not to love?
In Ant-Man I think Vernon should have a small role, but it'd be an important one that brings AIM and Ant-Man/Wasp on a collision course. He's the driving force in the story, and as such you need a strong actor such as Donald to deliver the story.
"Wait, isn't Starr Egghead?"
"You can't make Ellihas Starr MODOK"
This is what I thought when doing this. To me, Egghead is not a strong character on his own. He's smart, but he has no powers, and no way to really combat Ant-Man on a level field. I think tying in Starr to MODOK makes sense, and makes the best of two characters that maybe are not the strongest on their own in one users opinion.
Jared Harris is great as playing an intellectual, and I think that eventually morphing Hariss' face to become MODOK would fit just as much.
And finally I have Radioactive Man, in the comics he stands for China; but like the Mandarin I think he could also be changed (not as poorly though) to stand as a representative for AIM. A product of Starr's who teams up with him to fiht Janet and Hank.
Byung-hun Lee has shown in movies such as Red 2 and GI Joe 2 that he can handle action and overall I just like his acting skills.