If you want to play online, go ahead and get the expansion, and download the 1.10 patch, which, if it isn't released yet, will be very soon.
Just warning you, Diablo II's part of Battle.net is packed with idiots who either spend 14 hours a day mindlessly powerlevelling their characters or who spend their spare time finding ways to cheat and duplicate the most powerful items. The rampant cheating combined with the fact that I only have 56k AOL means that I haven't gotten much enjoyment from online play. Without a relatively high-speed connection, you will attack an enemy, and they won't get "hit" until 1-5 seconds later. Extremely irritating.
I found it fun to play through once on single player (Sorceress, mostly Thunderstorm and Firewall, difficult in the beginning but a juggernaut once you get to level 35+ with some decent equipment,) but I'd advise thinking twice before sinking the time and possible extra money (Expansion) on this game.
EDITED TO ADD: Playing Amazon? Stock up on the critical-causing passive, as well as the defensive passives. Offensively, look to multi-shot for crowd control and the excellent Guided Arrow for single, powerful creatures. Prepare to do a lot of hit-and-run fighting, especially in Nightmare and Hell.
Tons of info on the game (as well as nearly every game ever made for just about any system) here: gamefaqs.com
Strangely, they seem to be down right now. First time I've seen that happen in about 2 years.
Feel free to send me an IM if you're having trouble.