Differential, not sure, possibly at 60K mi
Fuel filter 60K miles.
Air filter 30K or 60K
NO timing belt, it's a chain, and no service interval for that.
Coolant every 3 yrs
Brake fluid every 2 yrs
There is a cabin filter that should be replaced I think it's every 45K miles
Spark plugs every 30K miles, no service interval for the leads (there is a lead or connector on each coil)
NEVER have heard of such a thing as an AC "regas" has someone fooled you into thinking this is a service interval? How about rotating the air in the tires too!
Transmission on that should NEVER be flushed, you CHANGE the trans fluid and filter every 30K miles.

No servie spec on cleaning the intake or plenum or like that.
Maybe you think the M111 should be a strong running motor. It does OK but it will never have the power of a 6 cylinder. But the idle quality may be an issue, I'd have to drive it. Are you used to 4 cyl motors? It's a good enough engine, just not a lot of power, just sufficient.

injectors could probably use a cleaning at that mileage...especially if it's only seeing 15 litres each fill..

AC regas I meant the freon in it was charged.
It's not that I expect a lot of power from it, but it just has some buzzing/humming vibrations even at 1,500rpm with new mounts which I think isn't normal and trying to chase it down

buzzing/humming at 1500rpm sound like exhaust hangers..ESPECIALLY if you have new mounts, because that vibration is then transferred to the next component, exhaust or trans/diff mounts.

Exhaust hangers you mean the rubber bushings which hold it up?
I will get them looked at, thanks for the advice.

The diff should be ok, as the vibrations are engine rpm dependent and not speed dependent. Also, the buzzing/humming vibrations are only felt with throttle on. With no throttle nothing is felt