You're going to be hard pressed to find an apartment in Chesterfield that you can afford on first year FO pay. Second, there's no easy way to get to Lambert by road. Third, I'd avoid anywhere near 40 because they're about to start a major project to expand it closer to the city but they're anticipating traffic backups way out west because of all the commuters.

St. Charles and St. Peters are good. Look north of 70, there are some better deals on that side of the interstate. It's also less densely populated and access to STL is easy via 370.