Syz,
I am about 15 months out from permanent seed brachy ( I dont' really know what the HDR stuff means). I too was concerned about libido which seemed to desert me right about the time of my diagnosis. I also got some mild ED from the treatment. I chose the seeds originally because I was told that if I developed Ed, drugs were more likely to work than they might be with surgery. Everything I've experienced and learned has supported that. Viagra works great for me even the Indian variety. In fact, other than the slight color difference, thery are identical in my experience.
I suppose the libido thing is psychological. Of course, I'm not a doctor, but mine has shown huge improvement in the last 3 to 4 months. In fact, to be perfectly honest, I just turned 60, and I am enjoying the best sex life I've ever had. Of course I have a very sexy girlfriend which doesn't hurt. Seriously though, she is a huge reason that I went the seed route, and I couldn't be happier. I have several buddies who went the surgery route and none are enjoying sex at this point. Not trying to start and arguments (that is pretty easy to do around here). Just saying that you made a great choice, and it will work out for you.
God Bless.