I plan to change the nivomat on my 2nd 780.
The first one used the IPD kit (springs and HD setup).
With tax, shipping and duties, the kit came in close to $1K CND
with the weak dollar.

For the 2nd 780 (and maybe subsequent 780s), I want to try to do it on
the cheap. Essex has Bilstein TC $94CDN while HD $177CDN.

The previous DIY posts talk about pressing the bottom
bushing out and pushing in the bushing/pin from the
outgoing Nivomat. Do you push the entire bushing out
or just replace the pin?

Which springs should I use? Again, previous posts mention
rear Toyota Previa or is there something with more ride height?

The springs I used from the Previa were perfect with regard to ride height. I don't know if they were from an AWD or a 2WD Previa though, but I do know they were progressive (variable helix spring). I don't know if they're all the same or not, but none of the posts I read on Turbobricks on the subject before I did the swap said anything about it.

Dale did the pin press thing on his Bilstein HD shocks, mine were pre-modified when I got the car. I think he only replaced the pin, but I think he explained this in a past thread you started - you could search for that easily enough. And you can also answer your question about the bushing by measuring the old and new pin diameters and the old and new bushing housing diameters & widths.

As for the shocks themselves I can only recommend the HD as I haven't ridden on the TC in the rear. The TC is definitely a good option for the front if you want comfort and control on standard or even IPD springs. The HD is very stiff in front, but is reasonable in the back.