I, like you, toyed with the idea of a server for a while.
Now I have one I would never be without it.
In response to your questions however:
a) this will depend more on your network than the server. The server will have no trouble, but having a decent wired network will provide the biggest advantage, wifi can be problematic with HD. (I dont stream HD, I have my HD films on my HTPC hard drive)
b) what do you mean 'remote access'. I connect to my WHS though the WHS console, which is excelent for what it does, though RDC for direct access as if I was sat infront of it (again, excelent for what it does), and also over the web using the web access to files, which is great, although Ive found little use for it.
c) I have uTorrent setup on my server, with Web Access enabled so I cn view and administer torrents and everything from any PC connected to my network though a browser. I also have a little application called Send2uTorrent on my PC that means when I download a torrent file it automatically opens it, sends it to my server and starts it downloading, which is great. (I also have a desktop gadget that monitors the torrent downloads aswell.)
While it does all these things very well however, the most impressive things about WHS that I have found are the storage mirroring and the backups. Ive got 2 HDDs in my server and it flawlessly accepts new drives, and ensures folders I have set to duplicate are on atleast 2 drives in case any of them fail, all automatically.
The backup side of things is the most impressive however. It simply astonished me. I wrote more about it
HERE.
Bottom line, they are great, get one.