At least you've made a good choice in the boat you want, and buying a boat at the end of summer should get you a good price since the owner probably doesn't want to spend money storing it over the off-season (that is, if Perth has an off-season). Your question has been answered many times, but I'll try to remember the boilerplate, and maybe some other regulars will fill in what I miss.
1. Check the hulls for soft spots or delamination, since this is very likely a deal-breaker. Others on this forum have reported fixing theirs, but be suspicious.
2. Unless you know for certain how old it is, it is strongly suggested to replace the standing rigging (shrouds and pins). Just do it.
3. Look for repairs made and ask about any you find. The same goes for corrosion.
4. Check if modifications can be un-modified.
5. Don't worry if the trampoline needs replacing, you can still get a new one. Hobie tramps are highly recommended for their durability.
6. As for the mainsail missing, that's not a big deal if you have the money to replace it. I don't know what the aftermarket for sails is like in Australia, but you can't go wrong with a Hobie OEM sail.
7. Check the jib furler for sloppiness or stickiness caused by wear, and if in doubt don't hesitate to buy the rebuild kit offered in the catalog. Having a smoothly working furler means one less hassle.
8. Have the owner rig the boat, and if willing, take you for a sail to see if anything is missing or needs replacement.
9. The Hobie 18 is a bullet-proof boat in many respects, and if it is well cared for it will give many years of service.
10. Order a Hobie catalog so you know how much you'll spend on parts replacements.
11. Make friends with your compatriots -
http://hobie-au.ning.com/12. I don't know anything about prices in your market, but a post-1987 boat in good condition is a rare find and should be strongly considered. And the wings make the boat even more desirable.
Here's to hoping you join us.