Hi and welcome to the Forum. If you're in Australia, shouldn't there be an extra zero on the price of a good used xs400?
Before you even think of trying to start it, drain
ALL the old gas out. Ideally, take the tank off and flush out the interior with some fresh gas. Then buy and install a $3 in-line fuel filter - again,
BEFORE trying to start it.
If you can do so in OZ, avoid ethanol gas.
Then, everything you do should result in improvement as long as you don't over-torque and break anything. The manual which you can download at the top of the GARAGE section here is for your exact model, so between now and Friday it would be good to read at least the routine maintenance sections.
Ask any questions on here that come to mind. We may all be geniuses now, but some of us were complete dummies not long ago. Believe me, my tag line (below) was earned!
Here's a great place to start - well worth reading before you jump ahead in your excitement and end up simply creating avoidable, owner-inflected maintenance opportunities -
http://www.xs400.com/forum/showthread.php?t=10595