1998 2.4?? Um this didn't do a 98 2.4 is problem no. 1
It will either be the 2.2 petrol, which is prone to bottom end issues if the service schedual isn't followed to the letter or the 2.2 DTI which is a great engine, just factor in £600 for a replacement fuel pump which it will be needing shortly, or the 3.2 which is a storming engine if it's been LPG'd.
Rust isn't an issue on the chassis, check tyre wear on the front, check for general offroad damage underneather. If it's a 98 is it a facelift 'b'? If so check above the plastic (!!) under body protection for any issues.
Check over as in a normal car, everything electric....
Get onto a bit of grass and check that the 4wd works properly, and remember to drive forwards and backwards to make sure that the auto hubs work if fitted.
Other than that, frontera's are very simple yet effective trucks, so checking one over is just above normal car checks.
