G'day maz.
Firstly Welcome to the forum and as always you will find your answers on the site .
Answer to your question ,The kid has basically summed it up when buying anything it is always a good idea to take along someone who has had a bit of experience and knows what to look for.
May i suggest a few things that quite often give trouble and are worth checking,you have already mentioned the clutch ,generally any tractor with a loader will show signs of clutch wear mainly due to the fact that the clutch is quite often Slipped when the loader is in use , a new clutch will cost around 600 dollars fitted.
Brakes ,the tractor has dry dual disc brakes which when properly adjusted work well.
Fuel lift pump 90 % leak fuel and 60 dollars will buy a new one.
Front axle assembly ,the centre pivot pin cops a lot of extra load with the added weight of the loader so if possible raise the front wheels clear of the ground and check ,wheel bearings,track rod ends,centre pin wear and king pin wear be extra careful when checking the centre pin ,the main casting has bushes or should have bushes make sure the pin has not worn through the bushes and into the housing this can cost quite a bit to repair/replace.
Cooling system,check ALL hoses ,water pump for signs of leaking ,water stains etc, fan belt and radiator ,have a good look and make sure it is not blocked with dust,grass clippings etc anything that could block it up and possibly overheat the engine.
The engine ,the 4.203 is as good an engine as perkins have made ,if like most engines it is looked after it is bullet proof,Note: perkins engines would be one of the cheapest engines to rebuild.
With the engine running and at running temperature ,idling pull the oil dipstick out ther should be little or no oil being pushed up out of the dipstick tube any oil coming out is a general indication that the engine is not the best.
You have mentioned that the tractor has been fitted with an external hydraulic pump, the industrial Masseys were fitted with the pump mounted out front,they had heaps of flow and ample pressure the only failing I have experienced is the drive shaft from the engine to the pump they tend to wear and rattle themselves to death so pay particular attention to the shaft,with the engine STOPPED get a big screwdriver and see if there is any play there.
The hydraulic system: the early Massey pumps were not known to be fast although reliable they are not really satisfactory when using a loader ,this should not be a problem so if you can put an implement on the three point linkage and have a play just to make sure the hydraulics do work .
last but not least, the final drive epicycle units ,they often wear and the outer axle seals will show signs of oil leakage, to check the overall health or otherwise with the tractor on level ground ,handbrake of select first high gear engine STOPPED stand facing the rear right hand rear wheel and push hard on the upper part of the wheel ,you want to ROCK the tractor sideways
If the drive units are loose and worn you will hear a clunk as the bearing slides from inside to outside,this test is a general indicator only and does not necessarilly mean the tractor is unserviceable it just lets you know what to expect in the future hopefully not the near future, do the other side the same way.
Brake and clutch pedal wear: again the brakes and the clutch get a good workout so expect the bushes in the Clutch and brake pedal pivot points to show wear, provided the bushes are not to worn they can be replaced for a few dollars.
Thats all for now.
Again Welcome to the forum .
Happy Days.
Hutch.