Run it on Maxol E5 with a good octane booster like Lucas or NF to prevent any serious knock due to bad fuel.
They run a fairly high compression engine for a turbo car and don't like low octane fuel at all!
Are you getting a guarantee with it? You should get a good full service done somewhere after you collect it and get a diagnostics check done just to make sure all the sensors are in good order, lots of people in the traders section very good at that work

The lambda sensor can be very expensive.
Check that the radiator is in good nick, I think they were a weak point on some of the early leggys.

Check that the brake callipers are in good working order, I think the 2 pots can sometimes develop sticky pistons after a few years.
Check that the idle is nice and steady when fully warmed up, if it's all over the place then it could be a sign that the MAF is on the way out.
The timing belt is due at either around 90K Km or every 5/6 years I think.
That's all the Legacy specific things I can think of for now
