I just started using Vista Home Premium, and I have had no problems at all. I was a little nervous about it, but my new PC came with it, so, I closed my eyes, jumped in, and I have been very pleasantly surprised.
I have played both Coercer and Dirge and they are both very fun. I have also played an Illusionist, and in my opinion Coercer can be a bit more of a challenge, as you have to keep your pet charmed, unlike Illusionist who has a built-in. I find Coercer to be the most unique and challenging class to learn - loads of fun! :)
Really what it comes down to is what YOU will find fun though. So try a few classes out and see what you like best, and go with it! I would highly recommend having all the expansions, especially KoS, for travel, if you and your friends will start out in different cities/factions. The spires will make travel to far regions much easier at lower levels I believe.
One thing also that I didn't see mentioned here is, if you and your friend do start in opposing cities, your first few levels will be spent on different starting islands(The 'get to know the game' area). There are separate starting islands for Freeport and Qeynos, and if you start in Kelethin there is no island at all, but rather a starting area directly within the world. In relation to the rest of the game this is but a small thing - just a fraction of the big picture, but just something that I thought worth bringing up in case you were not yet aware.
Whichever way you go, good luck and enjoy! :)