Has it been left outside some time? If so the ports on that engine/turbo have been open to atmosphere and done no good. You know Land Rovers and axles and gearbox may be usable, however value is as @Dippypud says in the chassis.
A couple of years ago I was driving my Ford Focus and was about 45 mins drive from home in the day when the battery warning light came on. I kept going thinking I would get back fine however after a couple more miles the electric power steering shut down [heavy like a series with a flat:D] I had...
To check. Use a multimeter set in dc volts across battery terminals will tell if alternator is still working. It may be the bit that cuts the power to the dash charge lamp has quit. Engine on [and rev it a bit] and off. See if it agrees with your dash meter.
All those bolts you mention came free on my truck with just penetrating oil. A good ring needed for those swivel housing to axle bolts. Some shock helps.
Heat is your friend with stuck stuff as your neighbour knows. No head left, angle grinder and vice grips with heat.
Mine having a good...
You have a need then. Your old alternator may well fix to be an on hand spare. Swapped out a non working alt for spare a while back and got around to sorting it just the other day. New rectifier and brush/voltage regulator pack for £20 + post.
If stator and armature windings are good often worth...
It is a shame so many folk, and not just farmers do not take basic care of their vehicles. A good clean underneath once a year and when dry a spray of preservative goes a long way. I was doing similar work on mine recently and all bolts that needed undoing did so. Thirty plus year farm truck.
You may have two issues.
If you are sure are not actually overheating with your thermometer readings 70 something at the top of rad and lower at the bottom hose is good. With engine hot and rad cap off [careful opening up, slow with thick cloth over cap] can you see coolant circulating? If so...
Petrol series known to cut out due to fuel vapour lock because of the overheating.
Are you losing coolant? Has your rad got all of its cooling fins?
A hand held IR temp reader is a good tool to check which parts of a cooling system is doing its stuff.
Brush pack and voltage regulator one combined on many alternators.
Unless you have some equipment on board with a heavy drain and in use for long periods on the batt there is little point in having a larger than standard alt. All it will do is charge the battery faster while loading the engine more.
Double check connections ?
With alternators the usual suspects are brushes and their contact rings, voltage regulator, and rectifier. How to check/repair alternators on you tube. This does mean removal and opening it up.