Single Player RtCW is pretty good just in itself - the AI is sharp, made by the same guy who did the Eraser bot for Q2. You can usually find the "Platinum Editon", which includes ET, for under $20 US. Even less if you buy it used.
If your at all interested in Fritz, I recommend you try it first - for all you know, there is no bot, I'm just connecting to everyones computers with my friends and pretending to be a bot!
EDIT: just heard of a new ET bot: CCBOT. Just came out.
http://lprins.free.fr