If it's a 1987 factory V8 it should be 3528cc with twin SU carbs although
these were never made with auto boxes so when the gearbox was retro-fitted
it may have had a later engine fitted, the options are 3.5 carb, 3.5 Efi,
3.9 Efi, 4.0 Efi, 4.2 Efi or 4.6 Efi. A leaky rear main seal is easy to fix
for competent home mechanic - Land Rovers are very simple mechanically.
BTW - it's not a Defender, that name was only applied to models made during
or after 1991, it's either a 90 or 110.

Straightforward - if you can fit say, wheel bearings you should manage this
OK with no special tools.

The auto was fitted after it left the factory but it is probably a 4-speed
ZF unit as the early 3 speed borg-warners are rare now. If it is anything
other than those two treat it with caution. I personally don't like
off-roading with an auto, but if fitted properly it will be as good as any
auto off-roader aided by the standard low gearing of Land Rovers. If you
want to swap it for a manual that is a simple bolt on job and can be done
while you got the engine and box split to fix the oil seal!

Assuming the engine and drivetrain are fit without noticeable leaking or
clonks when driven, check there is good fresh oil in the swivels ( chrome
balls behind front wheels) and that they are shiny and not pitted. Check the
engine carefully as being totally aluminium it needs a 50/50 mix of water
and corrosion inhibiting anti-freeze all year round and regular oil and
filter changes. Check steering for play - an inch is OK - anymore and the
slack needs adjusting out of the box, which is fairly simple. Check steering
box and pump (if powered) for leaks. Check the chassis carefully for damage
and rust - they are very hard-wearing but any if any of the outriggers have
holes they are easy and cheap to replace, but the main rails should be
solid. Body work is up to your tastes - virtually all the panels are
aluminium and bolt or rivet on cheaply, but the bulkhead must be in good
condition (the bit below the windscreen between you and the engine which is
steel!). Check the operation of the transferbox - the vehicle always runs in
4WD, there is no 2WD option. With the lever in the bottom right its in high
no difflock, bottom right id high with difflock, top right low no difflock
and top left low with difflock. Don't drive on grippy surfaces with difflock
engaged. Difflock light should illuminate immediately when selected and go
out after a few wheel revoltions when deselected.

Sounds reasonable to me.

Sounds like it should be fun - if you've any more questions post them in
alf.fan.landrover where many people will help you.