Agree with all comments above, the chap with the website has some useful info on the car check it out.

Get as much history on the car as possible, would also look at replacing the cambelt if it hasnt been done. With all Mk1's check the pollen filter is in good nick and isnt damp,the location under the windscreen scuttle a major design fault with the Mk1.
Dont assume the rattle from the area of the Cat is just the heatshield, try giving the Cat a light tap too, this caught me out when purchasing mine, I removed the heatshield thinking the problem was sorted only to find it was my actual Cat that was breaking up.
As stated the car needs to be worked hard to get 90's hot hatch performance out of it, also check the control blades on the rear suspension set up are straight, some people reverse into kerbs or similar and these are prone to damage. Check all electrical switches work and the central locking is in good order, check all doors lock ok.
If test driving the car try turning on full lock and listen for any scraping/rubbing sounds from the front end.