Hi Michelle,
Garrett's copal heavey medium works well for accelerating drying. I've used it and like it. It Creates a beutiful shiny paint layer. If you call, Ron will talk with you about how to best use his products, and answer any questions you have. he's a very nice man. He will also probably send you a test sample if you asked for one. His copal products are very highly regarded.
http://www.garrettcopal.com/prod_hist.htm
The umber and p/b sounds real good too, especially if you don't want to add medium into your process. Sometimes I add a little OH titanium white (doesn't contain zinc white) into flake to make it more opaque. It sort of splits the difference.
Studioproducts.com sells High Quality mediums and materials and provides a forum with loads of info.
The more mediums I try the more I like to just paint with paint. It seems the only real concensus I've seen regarding the achival quality of oil paint is that anything added to the paint reduces it's archival permanance. Maybe Richard can share his knowladge on this point as this is just what I have read.
I hope this helped a little.
Peace,
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