Hi zagbar, as you can see this forum is not very active.
I'll try to explain something but it's very long time I don't mod Ultima.
As you should know Ultima is made by a Client, which is the game itself, and a server. The server is SunUO (I like to be politically correct so I should advise you there are many other servers and two of them are more active than SunUO. Anyway even if it's a bit old SunUO is the better in my opinion).
For the client part you may mod the map (I'll give you more info below), the monsters, the game UI and the items appearance but since the game is old and so it's engine you cannot make monsters/items/gui with a better resolution of the game's original (640x480). I hope you understand.
For the server side you can modify anything related to the game design: spell system, spell book, quests, combat system and so on.
For each kind of mod you wish to do there are a lot of programs and as far as I remember this site:
http://uo.stratics.com/ is the best resources. You'll find a lot of things on it.
For server scripts start look in SunUO code (the script directory) and ask for specific issues here.
I'm sorry I can be more specific but I'm at work now

and cannot make a more helpful explanation.
"A spell, once casted, can be undone. But a secret, once revealed, can never again be kept."