Well, you picked a good army to start
You should definetly pick up a Grey Seer, a good way to do that would be to get the plague furnace/screaming bell kit. Build the plague furnace and then mount the seer on a seperate base. It takes a tiny little bit of converting something for him to stand on, but is a great model.
Also, converting one of those warlords to a BSB is a wonderful idea, BSBs are basically neccesary for Skaven, and you will rarely if ever want 2 warlords in an army. Other than that, run the clanrats with shields and hand weapons as slaves, and the ones with spears as clanrats, and generally I would advise against hordes - the steadfast is much more useful.
More plague monks would be helpful if you are running a furnace, as that thing'll draw every bullet, arrow, and suicidal goblin on the table towards it. The rat ogres are at a good size, but the 6 giant rats will do basically nothing unless you get more of them - there are a few on the plaguemonk sprue and you can get a good number of cheap ones off of ebay.
Jezzails (in sufficient numbers - 3 or 4 will do zip) and WLCs are good options as well, as are gutter runners. A word of advice - buy a box of night runners and run those as gutter runners, its much cheaper and honestly looks better.
And remember, if you can't decide what to buy, MORE CLANRATS. That is all.
Hope this helps, and good luck with the Skaven