If I import an existing level 5 character and start a new game in Baldur's Gate Enhanced Edition, does the game scale to match my level, or are the levels of the enemies I'll find in each region fixed? Will I have a "head-start" by starting with a level 5 character who has some basic gear?
The content itself is not scaled. However, XP gains are scaled, so in time, the game will 'catch up' to you.
Furthermore, I'd highly recommend this course of action. (Though maybe only level 3 or so), because getting one shot by wolves in the first area in BG (and indeed, in any low level D&D game or module), is one of the most singularly stupid and frustrating things in gaming, because extreme-low-level D&D is terrible.
The other answers are not completely true. Some of the random encounter spawns do scale with level. This means you might have to fight 10 Xvarts instead of two, for example. The fixed encounters (which are usually the challenging ones) are fixed. This become more pronounced in BG2, where depending on your level you might have to fight Adamantite Golems instead of Iron Golems. There are also some areas (e.g. Spellhold library) where a lich spawns if your level is high enough.
One other thing that scales (in discrete steps) is the level of the NPCs you can recruit into your party. But scaling helps you, so it's not really content scaling.
Also please note that due to the exponential nature of XP in BG, redoing all the content up to a particular point usually results in one additional level. This will not help you as much. If you have trouble I would recommend a heavier use of consumables.
Summary in bullet points:
- Some (random) encounters scale a bit
- Usually not enough to offset your higher level
- Especially in BG 1 it won't help you for long, since XP requirements scale exponentially
Restarting with an experienced character also causes some logical inconsistencies, like duplicated items that should be unique. You can also abuse the stat tomes. I think BG is a lot more fun without restarting like that, but since it's a single player there is no real foul ;)
I can't say for sure for Enhanced Edition, but original Baldur's Gate games didn't have scale in any form, all content and enemies were placed statically.
Since EE is more a restoration type remake with all core game elements unchanged, I guess there would be no scaling if you start with a 5lvl character.