LOL. well done, star performer.
Apart from the split in the drivers seat those Sonata seats look in good condition. I would keep the velour seats, more comfortable than leather. If you do change them, I could probably find a home for the covers.
An Hp-20 wouldn't be an easy installation as it is electronic and was never fitted to diesels. Its also fragile compared to an Hp-18.
What do you mean by funny changes of the gearbox? Hp-18 are road speed governed so won't change up until a minimum speed for each gear has been attained and change down once road speed falls below that speed. e.g. 4th gear and torque converter lock-up won't happen below 80Kmh, irrespective of how lightly the accelerator is pressed. This makes them rather buzzy when tied to a diesel engine, but does mean the gearbox is always in the right gear for acceleration.
The shifts are controlled by the pressure cable and the governor. Governor seals are known to split causing unusual shifts.
If it doesn't slip when in gear the basic gearbox is sound.
The auto box does make the engine consume more fuel, but they are very good on a longer distance drive.
A 2.1Td shouldn't emit visible smoke, they are a very good engine.
You can't fit a trip computer, the engine control is mechanical, no electronics.
Just drive it, put diesel in it and enjoy the drive.
That colour looks like Triton Green (Ert). It's virtually the same spec and must be a close Rp number (94 2.1SD A.RP 6218 ERT Triton Green), as the green one I had, except yours has climate control and presumably no sunroof. All UK Xm to 1998 came with a sunroof...
Looks like you're filling up a the lorry pump. I used to as the fill rate is much faster than standard pumps. When filling with 80 litres on a cold night, one wants to get back inside as soon as possible. Many cars cant accept the high fill rate.
Now I know which alloys to put on the D12 