Doesn't matter if the game came from PSPlus, was bought online or in a store imho. The PS5 will have backward compatibility and I believe Sony have said they expect pretty much every game to work. So chances are you'll be fine.
Back to ACC I am tempted by the game and pre-ordering makes sense to get the first DLC pack free.. but as others have noted there is zero news anywhere about it, not a screenshot, preview video, nothing. No news of a next gen update either - as per comments above pushing console versions a bit more based on PC could surely be done, but are Kunos going to spend the effort doing it ? It all just feels a bit lack lustre.
If you pass up the pre-order and wait to see how this turns out you lose 4 out of 15 tracks, if you want those and the GT4 pack when it comes out you're probably looking at £70, maybe more..
I think Kunos need to be doing a bit mroe on the PR side..