As MaxChlan linked, that is my KA swap article. Being as my truck is 2wd, some parts of the guide may be irreverent.
It is a rather large job, in the sense that the modifications that are required, are rather complex; namely the oil pan issue, and how best to undertake the job. If I had the money, i'd personally do a dry sump setup, but I'm sure we all would. I fabricated my pan from the KA and the bottom of the pan using a KA24E pan off a D21. You can use the Z24 pan, however it'll take some work to fit as a rear sump as it's not as long, and you'll still require the KA pan for the overall bolting pattern.
In terms of parts - new parts are *generally* available at Nissan dealerships, aftermarket options are copious. Enjukuracing is a good example of what's available.
If you are going to turbo it, stock can handle roughly 7psi, anymore than that and you'll want to start looking at forged options. In addition to this, the stock MAF from the S13 does not like boost, you'll need to find a MAF from a Z32 (300zx) and run an aftermarket ECU.
As for maintenance, the KA24DE is a rather solid engine, having run it for nearly 2 years now, I can't think of anything to complain about personally. I also know a ton of people who run them in their 240sx's for drifting and running n/a or turbo'd they're a hell of a lot more reliable than SR's from what i've seen.