Hi Bladyblue
I am actually working on a medieval themed RP region too and ran into some of the same problems as you. I am taking a slightly different approach to those recommended above, though those are also excellent resources. I am good with the basic building tools and can make my own structures, so thats no problem. For sculptmaps and textures, I have approached creators in secondlife and explained what I am doing, and they are often happy to send you an email with a zip file of the sculptmaps and textures for a few extra bucks. Many even offer "extended licences" for other virtual worlds. Texuturesrus.net is a good place to start. I have found many treasures there, and Morphe Inc let me use his sculptmaps at no extra charge. I advise you ask any texture/skin/sculptmap maker. Anyone who will have their stuff on a hard drive is worth asking. Worst they can say is no. Some will treat you like the anti-christ

, but thats cool. Just say thanks and move on.
Its such a shame LL dont let people back their creations up, as many builders I have spoken to would also offer this if they could, but even this is changing with the introduction of mesh. In-fact, if you have any self-made builds, a great way to export the basics is with the mesh workshop scripts available on SLmarketplace. It exports basic prim shapes you make in world as optimised mesh ready for re-upload into....well, wherever you like

Sadly it cant handle sculptmaps or severely tortured prims (yet).
Trees are a problem. Well, good looking ones anywho. I have not found any that come anywhere close to SL standard. Though I found some nice mesh bushes on CGshare.org that seemed to work well.
For mesh components, I have also been scrounging around googles 3D warehouse via sketchup and I must say I have found some gold in amongst all the awful stuff

. Uploading it is fairly simple, and you dont have to worry about prim cost so much here

so you can afford to upload now, check it out, find what you like and optimise later, if at all. Just make sure whatever you import is open source licenced! (most all of it is). If you plan to sell builds, you need to make sure it is also commercialy licenced. I can walk you through the process if you need, its pretty straight forward.
As for HUDS and RP stuff, I know someone was working on a system on OSgrid, but I dont know much about that. Maybe check out the forums there. I also know Ener Hax has some very interesting NPC scripts posted on her blog that I will be having a play with. thats over at iliveisl.com *EDIT* the NPC script is from Ohn Lang for anyone reading this after the fact. Sorry Ohn!
Well......thats pretty much all I know

hope it helps.