Other than the afore-mentioned rust, If it's a V6 take it out and make sure you rev it above 4.5K, if the battery light comes on she'll need a new alternator....pig of a job!
You've just reminded me, since no one else mentioned it...
Your prospective car assuming she's a V6? Drive her from cold. Insist on it. When the temperature needle gets up to the middle, your water is up to temperature. Maybe stop her and smell what's still puffing out of the exhaust. There shouldn't be oil smells, but she might be a but...fuelly.
Now go for a blat and get the oil up to temperature too. A good ten minutes. If she starts sagging after 3750RPM, its Inlet Manifold Runner Control (IMRC) is screwed.
The Duratec V6 is very robust, but not cheap to fix properly. If she's burning oil out the pipe, walk away. But if she's just throwing it overboard, that's not so much of a problem. Stoopid gaskets.
If the IMRC is screwed, (lack of performanc over 3750RPM) just ask for some money off. It's fixable.
But if it's smoky, sitting in a patch of its own sauce or noisy, forget it.
My reasoning is this: these are very cheap cars and they shouldn't give you any stress at all if they've been looked after. A dodgy one should be very obvious.
) has mentioned, most important thing to check is the state of the sills, floor and rear subframe - all are prone to rust. Get a solid one and you're sorted. Everything else can be sorted with a weekend of bacon, tea, beer and UKC folk 
