You don't HAVE to have TotSC to play BG. All what it adds are really only some areas, which are not bad at all, but still, not too many areas. It does raise the "official" level cap some, but it's still pretty low, rather use some XP cap remover. Baldurdash had some nice one..
It adds some new gear and such too, but in the end it wont matter that much. I've played through BG few times already, before TotSC was released. My "BG1-experience" sure was not reduced because of that. I liked the TotSC as add-on, it explains a bit of lore (about Balduran) and so on.. But no, you do not HAVE to have it to enjoy BG1.
What I suggest to do - play BG1 as it is now, play it all the way until you have to go to Baldur's Gate city. Meanwhile at the same time search for the expansion. When you get it, install it and done. Do the TotSC areas before you move to the final stages of BG1.
One bit of advice though - IF you have BG1 without TotSC and patch it, and later add TotSC and patch it too - your game will not run. It will give some error and that's it. ONLY thing what helps here is to:
1) Backup your savegames in some other folder;
2) Uninstall patched BG1;
3) Install BG1 again, NO PATCHING!
4) Copy savegames back;
5) Install TotSC;
6) Download and install TotSC PATCH;
7) Well, add also Baldurdash fixes, I always do;
This will ensure that your BG1 + TotSC will work in the end, it also will ensure that your savegames are converted for TotSC and will also work.
And other then that, have fun. I myself at times install BG1 + TotSC, create a char and then play with it all the way to ToB end
