Series Landy's and early Ninety and One ten models had their hubs lubricated by oil that migrated from the diff housing. On later models extra seals were added and hub bearings lubricated by grease. Some remove a seal that then allows oil to migrate to hub. My 1990 Ninety has the older method...

I had my first land rover in the early 70's [2a diesel] By late 70's folk were fitting V8's, Recon engines came mostly from crashed/ wreaked Range Rovers.

Looks like a bit of black Rhino Gaffer tape. E bay.

Four years is a pretty short lifespan for any battery but as @Knappster says above a small batt will not last. However OP says batt came from an Agri store and probably type 663/664 batt. I usually get at least 10 years use from them.

But it is good to "exercise" the high/low diff lock function now and then to keep them from seizing. You never know it may snow sometime even if you do not go off road. Do it on loose surface of grass or gravel.

Had the same thing with an Isuzu crew cab pickup [ big brut] delivering a prefab field shelter on an Ivor Williams 3 axle trailer to folks down the road. It could not get but a couple of yards into the field and the driver had no idea how to disable whatever system that was preventing it going...

A petrol series is in another world when it comes to noise, you will hear the tires on the road, wind in any gap, rattles and clonks from underneath ect. :D :D

Do check the voltage out put of your alternator, re my post above. Some years ago a mate's car was going through battery's every few months I tested his alternator at usual driving revs and it was putting out 18 to 22v! My Landy like yours is usually on short trips and in winter batt gets low...

It does sound like a mechanical issue. Where did they put the WD40? I am guessing the wiper spindles [ axles the wiper arms go on and the spindle [ axle rod] goes in through the bulkhead. ] Sounds as if wiper motor is still trying as it makes a noise [hums?] No noise likely fuse gone. Do not up...

The 2.5 is based on the later 5 main bearing 2.25 68HP unit and while not exciting in power in good order will give about 10hp more and about 15 more torque LBFT. Nice thing it is pretty much a straight swap. A petrol 2.5 will be about 83HP 133 lbft [ about 18 more HP more than 3 bearing unit]...

No harm in checking level. Getting the plates covered again if level low may extend life. I would check the output of your alternator. Anything more than 14v and that starts frying battery with the sort of symptoms you have.

Are you sure your motor is dead? Often it is seized wiper components / blown fuse/ switch issues causing problems.

Better top hose bleeder available on e bay

Very likely air in system if you have not bled it.

Does not look like any thing I have seen on a series 3.

Power supply to my place, err not for a while I think.

A good thing with the oil bath filter is if you go very deep and water does enter the filter is the water goes to the bottom of the filter and oil stays on top, so the first thing going into the engine is mostly oil and it gets burnt. Lots of blue smoke lets you know. Ask me how I know this.

Use Land Rover more often, keeps pully's clean. :)

You are not talking about squeaking when holding full lock are you?

About half is right for most coolant tanks. When I fitted a 200 into my 1990 truck I fitted the plastic tank for a Defender. Don't know if it matters but has greater capacity.