If you're going to get this game, consider buying the Humble version as it is DRM-free (it comes with a Scheme key too, if you feel you just have to keep feeding the monopoly more money).

Note Planet Alcatraz is a 1C game and they apparently hate GOG users, evident in the handling of the Ancestors Legacy NON-release.

Long RPGs with good story and replay value for weak computer are definitely: Geneforge is perhaps my favorite, I've beaten each of them at least twice with different character builds, different factions to support and different choices. I had a blast playing them.

Sounds like UnderRail might meet your requirements. https://www.gog.com/game/underrail

At least got the potato-PC thing covered. $15 now but past sales have seen it dip down to $6.

That price data courtesy of Yepoleb's "GOG Database": https://www.gogdb.org/product/1447688349

I back Spiderweb Software games as well. They can run on anything and they have content aplenty.

As mentioned just above Underrail is nothing short of great. Like Fallout 1 and 2... but better. Much better. And I like those two!

I got all 5, but not played yet. which one is the best to start with?

They are mostly independent with just a few references to previous games in the series but I would still advise playing them in order. The first one has no age related flaws and it's a good idea to experience the whole story. It is nice to see that the dev was still able to add something new and interesting every time he made a sequel.