If you want to get older versions like 2006 or 2004, you'll have to look for a brand nrw sealed copy on eBay or something. I've got a few on my watchlist but they are a bit expensive so I'm not gonna bite yet. I also suggest making sure that the copy has a return policy or a money back guarantee, just in case you can't register the serial number.
Once it ships, before you do anything go to the Trainz Helpdesk (search it up, it's the one powered by...Kayako?), and submit a Support Ticket asking N3V to check your serial number to see if it has not been registered yet. If it has not, you can safely put it into the Trainz Portal page where you register it and now you can access Content Manager and (maybe) the DLS. This is all what I've heard though, the only older version of Trainz I have is really TS2009, and that's just from the free Newsletter promo. CM doesn't work properly in CrossOver so I can't really use it. If it has been used, I'm pretty sure the software can't be used...or if it can you're limited to just what's built-in and you have no access to CM, I'm not sure though. I'm more confident that you just can't use it.
The "painter" program is PaintShed, and from the YouTube videos I've seen it's very primitive, only allowing for the built-in templates. You could probably do a better job of reskinning yourself using a program like GIMP (but I'm not gonna pretend like I'm a good reskinner...I'm ok at it). Not sure at all if it will work with Windows 11, and from what I've heard 2004 and 6 do work ok.
Make sure you get the patches for the games, TRS2004 was originally Build 2.0 (668), so make sure you get up to 2.4 (2365) by downloading SP4. TRS2006 started as 2.5 (2761), but I'ce heard the original release is super buggy and that when patching it to SP1 it needs to be as you first install it. Final build number is 2.6 (3092), I believe you can find the patches online. More info on the build numbers can be found here:
https://online.ts2009.com/mediaWiki/index.php/Build_Numbers
There's also some demos for UTC, 2004 and 2006, you can find them online alongside the patches and you can test them to see if they work. For me, UTC and 2004's demos don't even boot up in CrossOver Mac, while both 2006 demos work just fine.
All the info I've given is from my own experience - I've got two threads, "Buying TS12" and "Finding TRS2006 Brand New, Legally" to my name where I've asked about this and have gotten some good info. I also suggest just browsing the Forums and looking at other people's experiences, that's what I've done...and still haven't snagged a copy of older versions.
Anyway, good luck on your hunt!