I'd put the 300D drivetrain in the 300E. Find a transmission from a 190D while you're at it
Seems like a better option, provided the 300E is in sound shape.
For the bushings, you can look at them.. do they look cracked/dried/etc?
Something to keep in mind-- what's easy for some people is impossible for others. Do you have the space, tools and time necessary for such a job?