At todays rates £100 is about $235 AUS. BUT, I wouldn't worry too much about converting prices, from what I've heard boards are a lot cheaper in Oz than in the UK, so a board costing £100 would probably cost the equivalent of £60 in AUS $ anyway.
OK, things to look for when buying a second hand board.
WAX - Be careful if the board you are looking at still has wax on - it can be used to hide a mulititude of sins, like cracked glass/ repairs etc.
NOSE - Check the nose of the board, sometimes the nose could have been snapped off and reglassed, I would avoid a board if this is the case. Also check that the very end of the nose isn't chipped - I bought a board once, only to find that you could actually see the wooden stringer on the nose (sorted it out by buying a noseguard, but it's still worth checking).
YELLOW FOAM - if you can see any yellowing of the foam beneath the glass anywhere, avoid that board - it means that it's leaking and filling with water. This is difficult to see through wax, so be careful.
RAILS - Check all along the rails for dings and cracks. Some cracks are OK as long as there's no yellow foam under them. If there are any repairs along the rails, run your hand over the rail to see if you can feel the repair. If it feels pretty smooth you'll be OK, but if you can really feel it a lot, then chances are it's not a very good repair anyway.
TAIL - Pay particular attention to the tail of the board. The same rules apply for repairs on the tail, but make sure that the 'hard' edge that makes up the tail is consistent. The tail is the most important part of the board so make sure it's not too damaged. Some people like to do the thing where they stand their board up by pushing the tail into the sand - if there are any stones/ driftwood etc this can cause a lot of damage.
PRESSURE DINGS - you may find some smooth dents in the board, particularly where your feet go and sometimes on the underside. These are OK as long as the foam isn't yellow
FIXTURES! - Check around the fins and leash plug to see that the glass is intact - a lot of times fins will have been glassed in again when they've snapped off.
Also, try and get the shop to throw in a board bag (or at least give you a deal - some places will throw in a used board bag).
Hope this helps in your search.
And, just to clear things up, I thought your avatar looked a little like Kirsten Dunst so I was just curious, I wasn't (just) being a dirty old man!!!