Making the vendors sell mana potions would probably be easiest for a newbie. There's a plugin in the Keep that will do it, otherwise you could edit misc.txt and do it yourself.Changing the allowable affixes for rares is as simple as looking magicprefix.txt and magicsuffix.txt, I believe there's a column called "rare" or something to that effect, replace 0 with a 1 to enable that affix to spawn on a rare item.
Skills are somewhat more difficult to change, depending on what you want done. Some changes such as casting time (spell delay) can be done through simple skills.txt editing, others may require hex editing. I've not done much skills editing myself, perhaps do a search of the forum for this topic?
The runewords, crafted, and uniques will be time-consuming but not too terribly difficult. Most of the info you need is already contained in the respective files, so it's just a matter of referencing the other entries, copy-paste.
Good luck, and hope that helped. 
[EDIT: Oh, and for the bnet question... you may or may not be able to log onto the realms depending on how much your mod changes, but I definately wouldn't advise trying it unless you've backed up your characters. You CAN however play a tcp game with someone else who has the same mod.]
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"Ohnosecond: that miniscule fraction of time in which you realize you've just made a BIG mistake."
[This message has been edited by FeistyGirl (edited 08 February 2002).]