A good idea bringing this up for input from the wider community. (Why didnt I think of that!?)
Well, you already know where I stand on the decision of trade vs. renovate. A few other things to add to this we didn't discuss:
In my experience, of the items under discussion, the subchassis is likely to add the biggest, most audible difference.
Power supplies. Going from Valhalla to Lingo probably made significant audible differences, but I could't tell you what they were when I did it. Changing the tone arm (Ittok to PU7) was undeniable and memorable. Upgrading the subchassis was a similar improvement in type and magnitude.
Springs and grommets have been improved a few times over the years. Again . . . By the time I got mine installed, I couldn't discern any difference, but these are not costly items, and probably worth doing unless. . .
Analog Innovations' spring replacement rubber grommets... This time the difference was audible and memorable, and an indisputable benefit. More beneficial than the . . . .
Plattamats, which are also unmistakable, memorable, and indisputably an improvement.
The motor. I replaced mine with what was described as "the new quiet motor." I take it it's the current, new improved official, genuine item. Once again, it came, I installed it, I'm sure it's better but, to paraphrase the Christmas carol, Did I hear what they hear? Ahhhh... No!
So. . . For upgrads that rise above the threshold of "I could hear THIS in my sleep, won't forget it, and will be able to explain it years from now" I recommend:
A good subchassis. Keel is most expensive, Greenstreet is more affordable, as is Sole. I dont know which is better. Cetech (which goes in and out of production) is not as good as Sole, but gob smackingly better than the stock item.
Next, in terms of impact, would be a vast upgrade to the tonearm--but that's no where near the same value for money as the subchassis, so I say hold off a few years on that.
Next, I'd recommend you go for the AI suspension bobbin mod, replacing the springs.
Next, the plattamat.
Now falling in the category of "Honey, can you hear it? I'm really not sure..."
The power supply: greenstreet also has an external PS that is well reviewed. And there are several other players in the 3rd party market providing alternates to Linn's own Lingo spawn.
Quiet motor: I get it, it's "quiet", you're not supposed to hear a difference.
Various baseplates--ahh, I got some acryllic thing or other--couldn't hear it. Read some debates about the physics of whether it's better to have nothing covering the bottom.
New springs and grommets (if not the bobbins). I didnt hear a difference, but I felt good operating on my own Linn. Had I known I could do it successfully, I'd have done a brain transplant on my first Linn --my highschool girlfriend of the same name. Irish, not Scottish. A hah!