Need more information about the house style and why you are removing the existing base. I grew up in a ranch-style house built in 1958, and the trim was about 3" with a shallow curve toward the top edges...very simple. It was wood, and stained.

Trim today can likely be easily matched to what you have...if you intend to stain, then it must be real wood; if you plan on painting it, then do mdf or pvc. And, crown molding just looks ridiculous in a mid-Century house...just my opinion...mid-Century was about sleek lines and space-age styling.

removing baseboard is tricky business, the plaster could be attached to the baseboard, so patching will be needed.More than likely the baseboard you have will be from thicker wood than what they sell today. I ripped out baseboard from a 10' wall of my 1930's bungalow, and it was a mess as well as a rude awakening, i.e. wood thicker, baseboard extended to the subfloor.

i like plain 1x4s for trim capped with a cove. something simple like that would probably be nice. plan on going to a nice lumber yard to find straight boards though

Use wood. MDF won't hold up for that use. Here's a link that might help.

Here is a link that might be useful: MDF