Agree mostly with Tony's post above...the 85 1.4 is a beautiful prime (which is what you say you're after). The 1.8 version is cheaper and I'm not sure how important that extra little bit of speed/bokeh is for you.
I love the classic 105/2.5 - razor sharp and inexpensively had these days. The 1.8 version is nice too, and I've seen great results from a fast 135mm although never used one myself.
I'd consider a 50mm 1.2 (or 1.4) just for the different focal length.
Best of luck! I loved my F5.