Watch the auto trans in the old falcons... A little like the heads - guaranteed to die...
For VT-VY commodores, watch the heater core, it's prone to corrosion

For VZ commodores, watch the sump, it’s lower than the rest of the bits and pieces at the front... Fortunately, if this is busted, you'll know all about it...
If choosing between falcon and commodore in that price range, I'd go a VT wagon (at your upper end – but expect new tyres, battery, rego, service etc etc). I'm not a fan of the rear suspension in the older falcons (vs the commodore) and the styling of the falcon wagon is
intersting
Oh and the driving, older falcons are like driving a couch. Commodores are a little more responsive