Telyn,
I think you may do better looking at the Tudor period for a representative economy - other than the Legion, almost everything else fits better as 15/16th century or a little later. Elizabethan/Shakesperean England would probably be a lot easier to research too, given we have so many more records regarding that time.
There are a couple of TV series you might want to look for as well - "Tony Robinson's Crime and Punishment" looks at the historical foundation of the English legal system from the Romans onwards, and "What the Tudors did for Us" looks at the impact the Tudors had on modern life, which is a good way to see which items may be anachronistic and which are just unexpected.
One thing that may take a little tinkering in the Oblivion engine is coins, and getting the correct "change" in the minimum number of gold, silver and copper coins. Also exchanging coins for higher or lower denomination with no profit or loss on the trade - unless it's "foreign" coins (do Elsweyr/Morrowind/Skyrim coins have a different value and need an exchange rate, hence buying and selling at different costs?)
However, if you DO decide on a medieval setting, I'll certainly give it a try - the economic changes and challenges would be interesting to say the least. I'd also love to try and play a Cadfael-type character - an ex-warrior cleric puttering about his herb garden, but also doing detective-type stuff on quests, which a medieval background would be FAR better for than the later setting. Maybe I'll have to learn modding myself... 
I thought about Tudor. There are a few drawbacks for me. First, there is a very strong ancient Roman theme going on with the Imperials, and it's a bit easier for me to deal with Roman traditions in a medieval setting than a Tudorlike setting. The technology level is a mix of eras, some even later than Tudor, but the production of goods seems more medieval to me than Tudor. It's complex, because it's hard to imagine how real magic might have changed Tudor society had it been a pervasive as is the case in Oblivion. The thing is, the astrology thing fits nicely, but it's not a big influence on the economy. The magic items in Oblivion are a huge part of the economy, and I have difficulty here working the flavor of them into Tudor. Oblivion has a polytheistic faith, an emphasis on nature, etc. Everyone in Oblivion believes in magic and almost everyone practices it to some degree. It's not formally scholarly. Originally some animals in Oblivion were meant to have spells. It does have some of the trappings of later practices, such as the alchemical tools. Hourglass wasn't common until late medieval, but alembics and mortar/pestle are much older. This is an example of Elizabethan era magic:
http://en.wikipedia.org/wiki/John_DeeSo there we do have alchemy, and astronomy, and summoning. However, the alchemy was a philosophical/spiritual endeavor and Oblivion alchemy doesn't seem to have much of that. It's less abstract and symbolic. In terms of economy, only the alchemy has a major impact, and magic weapons and armor have at least as much. I think that's why I am leaning to loose medieval here. You may be right though, it might turn out to be much more workable to go with a later era.
Love Cadfael. The character I am playing most right now does mostly putter around in an herb garden and occasionally solve mysteries, though she apparently also feels it is her duty to do the religious type quests. I am not really fond of the main quest at this point, but this character is insisting.
I guess I will just play with pricing and see where it goes. Once I have a better idea how it plays in game, everything I am currently thinking of doing will probably change. The Local Economy feature in Enhanced Economy might simulate the exchange rates you're wanting, indirectly.
http://www.tesnexus....le.php?id=25078