I have not done this before on the era van that you have, but I have added power mirrors to a 91 van. It is only my suggestion to NOT do this UNLESS you have a donor vehicle that had power windows.
There are many things, like the wiring harness, relay module, switches, and then not sure if you will need brackets to mount the power window motors to.
Don't want to burst your bubble, and understand you wanting to add the feature, but there are a few things that you will want to look into (see just a few mentioned above)
Good luck though.....