"Would It have to be a toroid or will a normal transformer do?"

There is no difference exept that the toroid is usually smaller than a normal El transformer.
Toroids also have a bigger inrush current.  In a preamp you won't be using a 300VA transformer so it's not a problem.

For a preamp the transformers are small enough to give you with both options open.

"Would something like this be useable. Or is a 9v powered unit
not enough output to be useable."

I recon a 9V preamp is more than enough, depending on the gain of the power amp.

Let's take a power amp with gain of 22 (a lot of gain-clones use this).  Power rails of +-25VDC.

Maximum output voltage of the amplifier will be less than the rails, lets take 24 as an example.  With a gain of 22 we get 24V/22 gain = 1.09 V input signal for max voltage.

Now your preamp has 9V, If you use op-amps you'll get 4.5 volt rails.  Your output only needs do be 1V thus you have headroom of 3.5 volts depending on the op-amp.

This is all really simplified I know but it gives the idea

Using 3 batteries will make is difficult to get symetrical power rails, two on one side and one on the other, so one rail will still be 7.2V.

If you use an op-amp to derive a centre voltage, you're limited to the output capability of the op-amp.

Try and use a transformer, the PC power supplies are noisy and just complicate things.

Try to source an old broken stereo amplifier and take its transformer out. You can get a rough estimate of the transformer rating by looking at the fuses used, the service manual (If you can find one) and the amplifier it was used in.

for a schematic, look on www.sound.westhost.com. It's a site run by Rod Elliot and he lives in AUS.  He has a 100W guitar amplifier and if memory serves correct, he sells pcbs for the guitar amplifier. I might be wrong but I think its 22 AUD dollars.

If you want to keep things simple go for a LM3886 chip from national, or something similar.
I'm planning on doing this because i've got a LM3876 lying around.

The most expensive thing that I've found is the speaker. Unfortunately you can build the best amplifier in the world, but if the last step is rubbish, you won't get a good sound. Save a bit more and get a nice (reasonable) speaker. The rest is easy in comparason.

Good Luck.