A workflow that converts to 3D using tooling that you know (Max is OK, or even AutoCad 3D might work) and exports some manifest of “asset name, position, orientation” that you can import into the world, would probably work best.
You will need to work with Datasmith and your CAD tool to figure out what the best way to get the data out would be. You’ll need to write some small amount of tooling for the Unreal editor to import your manifest and turn it into level actors and assets, potentially updating previously-imported actors/assets.
Finally, for workflow, you may want to split your world into sectors, where there’s a separate manifest for each sector. Some tag on the original data that you figure out would tell the system which object goes into which sector. That way, you can separate each sector into a separate sub-level/map, which is great both for level streaming, source control, and general sanity.
Beware that going from 2D footprint to believable, good looking, varied 3D models is a lot of work – potentially significantly more work than the initial work to make the 2D footprint outline (assuming that’s what you’ve made so far.)