Well, we all gotta start somewhere right?
First thing's first - in this game, skinning and plugins are two completely separate entities. To put it simply (if not almost entirely accurately) plugins are for creating new UI elements, while skinning is to modify existing ones. So, as you probably can figure by now - a plugin manager isn't going to help you much, as almost everything you listed would be handled by skinning.
The lone exception would be fonts - there is no way to modify those for existing elements.
Skinning is essentially XML based, and a nice example covering most (if not all) skinnable elements is included with the latest skin pack. This pack not only contains an example definition file, but the majority of images the game uses, so you can use those as an example for creating your own graphics, and change size/position via the XML skin definition. I do believe there should be some kind of app out there that may assist you with showing a preview, but to be honest, I don't know for sure. My only experience with skinning has been to assist me with other projects - so hopefully one of the more experienced skinners can give you more information on that.
Now for bad news part 2:
The latest game update introduced at least one new window, and a major redesign of the character panel. Unfortunately, Turbine hasn't yet released an updated skin pack - so for right now, skinning the character panel and wallet is iffy at best. However, there should be nothing stopping you from modifying the other UI elements in the way you describe.
Hopefully someone with a bit more experience with skinning will chime in and fill in what I'm unsure about or didn't mention. But, at any rate - I hope this at least gives you a nice overview