My boys showing a lot of interest in his big sisters bike now.

I know there are other makes but, looking at the Islabikes website, I wonder which would be best to get him.

He's two and a half, so the balance bike is the first one I looked at, then I wondered if he'd outgrow it too soon. He's only a couple of cm off both the minimum height and inside leg measurements for the 3+ bike (Cnoc).

What about just getting that and taking the pedals off to start with, or is that a bad idea? If he's going to get a couple of years out of the balance bike I don't mind spending the money on that.

you can get balance bikes for £40, they're not as high standard as the islabikes ones, but will do the job for as long as he needs at a lower cost.

Buy the balance bike and flog it when he's outgrown it. You'll easily get your money back on it.
Don't put him on a bike that's too big too soon and avoid stabilisers.