There is some misinformation that seems to be rampant particularly within the GOG circle that I'd like to call out:
1. No, the game is not fixed. It's playable, borderline, but performance is definitely not up to standard even compared to other games from that era. The GOG version is probably the most stable one and there is a mod for it called the Juiced Patch that will fix most performance issues and allow you to enjoy the game properly, however, that patch does not ship with the GOG version out of the box, is quite CPU heavy and does not fix everything, so claiming the game "works well" without mentioning that is misleading.
2. No, the DLC has not been ported to PC, the mission scripts are present in the game files and there is a mod that allows you to play them, but the actual content and assets that makes them unique is not available in the PC port in any shape or form.
3. A re-release is not in the works. There was a semi-official patch that was being worked on by a few Volition volunteers to fix the port properly and bring the DLC to PC, however, it's development has been such a monumental shit show that it's unlikely anybody feels motivated to work on it anymore. The lead developer on it died a while ago, the second-in-command managed to get himself cancelled by the community, the developer of the game was shuttered some time after, and the publisher is one foot in the grave as well. So needless to say everybody involved has bigger issues to worry about than fixing this game. Don't buy it expecting that to happen.
That said, game is a broken masterpiece. You're basically gambling on whether you'd be able to play it or not, but if you're willing to roll the dice this is hands down one of the best open world games ever made.
For those who don't know, it was CD Projekt who fumbled the PC port. If you decide to get it, kindly remind them of that every once in a while and maybe they'll decide to do something about it before the source code is lost again.