You've got a good start here.

For stairs, etc. you could check out the Dundjinni user creation forums. Lot's of great stuff folks have created (and all free). All you need to do is download the appropriate piece(s) (I recommend firefox - for some reason IE downloads the .png as .bmp) and bring them into your GIMP map.

Have some 'rep' for uploading a map in your very first post *bonk*