Ooh, this again. There's a lot of heated debate around the internet about which version is "better". Obviously most people on here are going to be those who prefer the original version of the game, so keep that in mind. That said, this is my perception of the arguments:
Advantages of EE:
Several effects come preinstalled which you'd have to install mods for in the original version (playing in the BG2 engine, and in modern resolutions, for example)
A small amount of extra content is added, although I've heard varying accounts of how well it fits with the original material
Some people prefer the UI of the EE: the split between preferring the EE's UI and the original's seems to be fairly even, so I'd suggest looking at screenshots and deciding for yourself.
Advantages of original:
More flexibility and choice with mods, since many mods don't work with the EE. For instance, last time I checked there wasn't an EE mod which let you play both games together as one game. More mods means more replayability.
Price: it only costs half the EE price, and less still if you get it during the sale that's ending today. Do bear in mind that if you want to play BG1 and BG2's engine you'll need to own both games: but I'd recommend getting both anyway :)
The UI: see above
The option to try BG1 in its own engine, rather than BG2's
Fair warning: personally I've only played the original, so don't have any experience of my own with the EE. Nostalgia isn't a factor, though, as I first played it in any length last year (and found the UI perfectly usable, once I'd got used to it).
To answer your second question, there are three different stickied threads giving mod installs. Which one you should follow depends on how you'd like to play:
Follow the conservative guide for modding BG1 if you want to play BG1 in its own engine, as it originally was. Then follow one of the other guides for BG2: they're similar enough that it doesn't matter which you choose.
Follow this guide if you want to play the games separately, but play BG1 in the (updated) BG2 engine. Follow this one if you want to combine the games into one big game, with the option to carry over all your NPCs' statistics from BG1 to BG2. You'll find a similar sticky for the IWD games in their forum.
Whatever form you play the games in, I'd recommend familiarising yourself with the DND 2e rules, by reading
this guide. It's shorter than it looks, as there's a lot of tables in it.
Hickory: Try them; don't rely on third party information.
The problem with that being that you end up spending $30, rather than $20 (or regional equivalent) or $10. While I agree that the games are worth that much...it is a lot of a price increase to just try them both out.
