There's a thread on here about what to look for when buying a Skyline. Can't remember where though, sorry.
They're reliable. Start modifying them and you can expect to reach (roughly) between 270-300rwkw before having to play with the internals.
There are parts all over the place for these. Hardest things to get are the original parts of course. But 99% of parts are easily sourced. And they're nowhere near as common as Commodores, they just stand out a whole lot more.
No common problems as far as I know.