It's incredibly interesting how these cars are aging - can't imagine it's too dissimilar to most cars though.
I picked my original 55 plate in 2007 for about £13,000 then this 06 this year (2023) for £8750.
In the paperwork I received with the car was the receipt from the previous sale, which was in 2019 for £5000
I can only imagine that these have hit the bottom end of the market already and should now at very least hold (hopefully) if not go up.
Trying to find a unmodified one is a task in itself.
When I was considering getting another EP3 I did the research and tried to get comfortable with where these are cars are aging and everything pointed towards rust repair and rust prevention.
Like you say, the main things people focus on now are prevention and maintenance. Removing the rear arch carpets is a good idea - although mine had already been treated and had the carpet put back in, so I'm inclined to leave it for now and keep an eye on it. Speakers, sills and all that is where most die sadly.
***I WILL NOT TRACK IT