'What I'm about to say is only speculation.' seems to be the closest to your general meaning, and is a fairly common phrase. The construction you used isn't exactly wrong, but this one is a little more elegant. Depending on the context you can also say: "This is only speculation, but..." to introduce a topic.
Some other options which are both natural and correct:
"What I'm going to tell you is only speculation, but..."
"What I'm telling you now is nothing but speculation."
"This is just a guess, but..."
Edit: ...also, 'naught but speculation' is probably what you meant to say in your last two sentences, and it _does_ mean 'nothing but', however it's very, very formal, and almost archaic (at least in American English), so you probably want to avoid using it.