Ice- If I were you, I would get the written out of the way ASAP....If possible, I would also try and take the CFII written around the same time you take the Instrument written because they are nearly identical....
Dont worry about the instrument flying...When you first begin, it will be the most mentally and physically challenging test you have ever gone through...I'll be honest...Even after flying only 50-60 minutes in IFR conditions, i'm ready to go to bed once I land! Your gonna be scared chitless at times...But once you get over the fear, your going to feel jubilance, and pride...I dont particulary like flying a full IMC flight...But when I get down, I think holy chit...Look what I just did! and its an incredible feeling!
Personally, my two biggest set backs during my instrument training were DME Arc's, and holding on NDB's...All NDB stations are going to be phased out over the next 5-7 years, but you will occasionally have to hold a DME Arc...
Just remember...ALWAYS BE CURRENT! And that doesn't mean just being current under the FAR's...Have a self currency mininum...Flying IFR is a heavy work load, especially when flying alone, and it's incredible just how much you will lose, even after taking a week or two off...
I dont think you are going to need an extra online study...Just buy an instrument kit, and all books included will be sufficient....
So good luck! It's well worth it in the long run, and dont get discouraged!
And Study the damn written, and get er done! It will make things much easier