Hi and welcome. A gearbox service certainly won't do any harm, usually around the £250 mark BUT make sure they or you put the correct jaguar recommend ATF in as "normal" ATF will wreck the box. As for the rattle, cam chain tensioners stick and give the diesel rattle you hear, normally fixed by giving an engine oil flush then a good quality oil put in. The only other things to look out for is rust, start with the sills behind the plastic covers then check very carefully the rear subframe for rust, unusually common on these cars but more prevalent on older cars. Then it's just things you would look at on a lesser car, like does everything work, aircon, heater blowing cold and hot. If it has satnav don't get excited, it's woeful and no upgrades are available. Window regulators generally snap the lugs holding the glass but repair kits are available on eBay. Aircon not working on earlier cars is almost always down to the pipe that runs by the left side of the engine as you look at it and is very expensive to buy and time consuming to replace. Sorry to paint a bad picture but some cars suffer badly from neglect while others take it in their stride. Enjoy yours and keep us updated.