I like running at high detail if possible, but I can settle for a little less. As for Guild Wars, I tried WoW and like it at first. However, I got really sick of the grinding, so if there's a lot of that in Guild Wars...
I like being alone in MMOs, even if that seems counter-intuitive.
Guild Wars is pretty much different from every other MMO out there, so don't use WoW to form an opinion.
For grinding... I've never experienced any. You'll reach the max level, 20, relatively quickly - heck, if you play Factions, you could easily reach 20 within a day or two of casual gaming, so there's absolutely no level grinding whatsoever. You won't have the money/skill points to afford every skill the moment they become available, true, but that's hardly a huge loss (you'll obtain hundreds of different skills easily enough, and since you can only use 8 at a time...).
Now, if you actually care about
appearance, there are a few grindy elements - 'better' armour has the exact same stats as the commonly-available stuff, but it
looks cooler, and that's enough to make some people farm for some items. Likewise, you'll find multiple 'max stat' weapons easily enough... but finding one that
looks awesome, too, is a bit more dependent on random drops. But if you don't care that you aren't wielding a sweet blue, flaming sword, then there's practically no grinding whatsoever.
And as for the last question... Guild Wars is probably the easiest MMO to play single-player. The whole game (outside of towns) is fully instanced, and if you don't have a full party (or don't have a party at all!) you can 'group' with NPCs in town, and they'll fill out your empty slots (and often do a better job than the
players - knowing that your healer has computer-level reflexes and a built-in order to prioritize you over the rest of the AI is nice.
) I've gone through every mission in the game with an AI party, so I can certainly attest that it's possible and enjoyable.
(Yes, I know, I probably sound fanboi-ish. Seriously, though, it's a great little game - better than most other MMOs, and the 'no subscription' thing means you can always come back to it later.)
If Stalker interests you, pick up the first one - not only because it's liable to be the cheapest, but because (AFAIK) it's got the best mods. Stalker Complete 2009, I believe, fixes a lot of bugs, along with other minor changes. Whereas Guild Wars is an MMO that can be played as a single-player game, Stalker is a single-player game that wishes it was an MMO - if you like wandering the vast expanse of a sandbox, it can be one heck of a ride.
And I've never played Just Cause 2. It looks interesting, but I'm running XP and am too lazy to upgrade.