I am going to view a friends 2004 Td6 next week with 77k miles from new, anything to look out for or be aware of?
Thanks

Gearbox
Steering wheel in and out motor
Bushes and suspension in general
Rust on arches/sub frame/rear door

Lots here in the wiki (top right of the site navigation under the FFRR logo)

Enjoy

Water ingress in the rear left side compartment.
Do the sat nav, radio, cd player and tv work?
Service history and list of work done to the car.
Does the door locking work correctly, how many keys?
Does the air suspension lift and fall as it should?
Does the front blower fan change speed correctly?

There will be rusty brake pipes - front, rear and connecting (all hidden away under plastic covers) unless your friend has had them replaced .
Experience is the only genuine knowledge, but as time passes, I have forgotten more than I can remember
Volvo V70 P2 2006 2.4 Petrol 170bhp Estate SE
MG Midget Mk1 1962

Previous: L322 Range Rover TDV8 3.6 2008; L322 Range Rover TD6 3.0 2002; P38A Range Rover V8 1999

Will that have been picked up on previous Mot?

No. MOT testers only report on what they can see, they do not remove parts at all.

This is why you will see.....

"Under-trays fitted obscuring some underside components"

In the same way they won't check a seatbelt where a child seat is left in.

The brake pipes are probably rotten and hidden, they use the above advisory to cover their bottom. 2006 4.2 SC
2019 Renault Zoe Dynamique Nav

But if the rest of the car looks sound, don't neccesarily let that put you off. The aren't a fortune to replace. There is nothing that can't be fixed with a hammer😜😜
FFRR 4.4 SDV8 Autobiography Santorini Black.
Fiat 500x 1.4 multiair Lounge 2015
2010 LR D4 Commercial 2.7 TDV6

Thanks all, I also need to weigh up the options between buying this with 77k and no gearbox work having been done or buying a higher mileage one which has had the gearbox replaced.
I do very little mileage as I work 4 miles from home so I might get lucky with the gearbox.

Diesels hate short trips, they are designed to be running all day long (hot) e.g. fairground generators.

If you're doing 4 miles each way to work buy a petrol V8. Or go electric.  2006 4.2 SC
2019 Renault Zoe Dynamique Nav

Cheers not keen on petrol option.