a) Unless you're looking for old stock on eBay or something, NWN EE is the only version you can buy today;
b) NWN EE also includes the classic version of NWN as a separate download (at least on GOG it does);
c) NWN EE didn't have much going for it at launch, but between the minor graphical improvements, the updated Darkness over Daggerford and the new Tyrants of the Moonsea expansions (both separate purchases!), it now offers enough added value;
d) Most if not all of the old DE modules should work out of the box on the EE, but you could potentially still run into the odd bug with non-standard scripting;
e) No must-have "mods", unless you mean must-play free "modules", in which case: Swordflight, Crimson Tides of Tethyr (prequel to TotM);
f) Yes, you can go solo. A Rogue dip might be useful, though;
g) As far as official content goes, the OC's pretty bad, but SoU, HotU, DoD and TotM are all great campaigns;
h) This thread reeks of bait.