When checking bottom end bearings, if you can feel the free play they're beyond rooted already lol. Plastigage or similar is the easiest option. Take the cap off, wipe the oil off the journal and bearing, then torque it back up, remove the cap again and compare the width of the plastigage to the markings on the packet. Obviously the bearing should be oiled again once you're done.
Off the top of my head the spec is 0.035-0.086mm with an upper wear limit of 0.15mm.
Have a look over the bearings while the caps are off too, deep gouges, worn passed the copper layer etc and they should be replaced.
SDs don't love a cold start, in warm enough weather they'll fire up with no glow plugs after a lot of cranking, but in cooler weather forget it.
Using easy start/aerostart etc won't hurt anything if its only used once in a while, but I wouldn't be using it all the time. They tend to knock pretty badly on first start with it.
Glow plugs shouldn't be too hard to track down, HKT, NGK, Bosch etc all have online catalogues.
From your video I don't hear anything concerning about the engine. SD's arent nearly as refined as modern diesels, they're very commercial old engines so they are louder and a bit more rattly than new stuff.
Sadly you can't feel for play at the caps unless they are REALLY Knackered!
You need to remove a cap to check for wear.
If they look ok probably will be. Any sign of wear and a bit of plastiguage is your friend.
These engines can be a bit noisy. Also yes they can be started without heaters.
Bit of noise, lots of white smoke but eventually they will run IF battery has enough grunt o keep turning it over.
24v glow plugs ones may be available from PatrolApart.com.au or ebay.com.au or similar.
As far as I can see the W/R wire on the wiring diagram (going to the two glow plug relays). (With relay 1 closed it gives 24v directly to the glowplugs. With only relay 2 closed the 24v goes though a resister first (reducing the voltage) then to the glow plugs.)
The W/R wire does not go to any other components, only powering the glow plugs, so cannot see any negitive knock-on effect to changing that from 24v suplly to 12v supply, to suit your fitted plugs.
Whether you choose 24 or 12v, its probably worth picking 1 and sticking to it throughout the vehicle as much as practical. Given that the engine accessories and wiring are 24v, that's what I'd be choosing, especially given the cold temps you are starting the engine in. Plus halving the amperage on the 'Prince of darkness' wiring where possible can't be a bad thing lol. Anything that is difficult to change, like the instrument cluster, can be run through a 24-12v converter.
As for glow plugs, I'd order 24v units, they should be available in Europe. Early SD33 glow plugs are 75mm long, later models used 70mm, but the resistor length is the same, so either can be used.