On my '89 240, the coolant level started to drop slowly as the radiator sprang a tiny leak out of the one of the bottom corners. As Mike says in his post, its about this age they tend to go so its not a bad preventative maintenance plan to include the rad. They are quite cheap (£60 for a good quality Nissen rad) and not difficult to fit. Gives the opportunity flush the cooling system as well.
Apart from that, I would recommend looking at the suspension bushes, even ones that appear OK and show no movement can be worn out as I find out. Again depends on how the cars been used.
Engine mounts can suffer, especially the one near the oil filter which is in a perfect location for a coating of oil every time the filter is removed.
Cambelt is often ignored in cars of this age even if they regularly 'serviced' so check that.
I also changed the gearbox and diff oil.
Also make sure the flame trap/breather system is clear - again often overlooked.
Hope that helps
Steve