It definitely sounds like you are well on the way actually. You have GIMP so you don't need much more than that. You can grab a lot of elements from both RPGMapShare (which you obviously already know about) and also the Dundjinni forums.

I would suggest that you actually start with the outer areas first so that you have an idea of size and placement that you'll need. It can just be a rough sketch with nothing fancy, but from it you'll be able to have the outlines of the various buildings that you will need to fill in as well as the scale of them.

Perhaps start with some simple shapes ovals, rectangles, etc and size and position them where you envision things being. Then go on the hunt for building images from the aforementioned sites and plop them down over the shapes. I think you'll find that the map will basically take on a life of its own and just grow as you see the available objects that you can drop down.

That would require the least amount of graphics know-how on your part. As for the arena, a similar process can be used. Start by determining how many pixels in size it will need to be and draw an oval to that size. Sketch in simple objects again to be the different areas within the arena. Using layers you can easily trace over these objects to create your walls and then drop in objects and textured polys for the different areas.

The key is to get the process started and then share your work here and we can provide a bunch more help and direction and tips to get you where you want to go.