You will hear if the chains are due for replacement they will sound like a jar with marbles in it, at that mileage if they haven't been done and sound not too good it would be wise to do the lot rather that just guides/tensioners, the extent if rust you might find could be hidden on the floor around the body bungs and you might not get to see how bad it could be until you stripped it down a bit. Obvious checks for leaks are at the crack pipe, t-stat housing and oil cooler, look for pink powdery residue at all hoses etc, the oil cooler may have an oil drip due to old hard seals, which are cheap