Regardless, if I was going to play the series, would it be best to install the expansions from the start, or wait until I'd beaten the game? Talking about both 1 and 2 here.
Install the expansions before playing. In BG1, the expansion areas are available from the start IIRC (but so are most areas, really). In BG2, one expansion area is available before you beat Shadows of Amn, though I'd avoid it until at least Chapter 6 unless you know what you're doing (being a newbie, you won't). The rest of Throne of Bhaal comes after you beat BG2. Note that you can install and play Throne of Bhaal
after beating Shadows of Amn, but I don't
think you can play Tales of the Sword Coast after beating BG1. So definitely install TotSC before playing BG1.
Also, the Baldur's Gate Trilogy mod (or Tutu, but I prefer continuous play to separate installs) probably isn't a bad option for your first playthrough. You'll get the improvements of the BG2 engine as you play through BG1 (plus a few graphical changes, though there are mods that restore those to their BG1 versions), including the
immensely useful TAB key (highlights containers so you can see where all the hidden stuff is -- and some of it is
well-hidden).