I know Daggerfall is scaled to at least some extent. Humans scale up in power, monsters have levelled lists where stronger ones spawn at higher levels, right?
Pretty much, but it's not unlikely to have monsters such as ghost/wraiths spawn at lvl1/2 (happened to me more than once), not to mention zombies, which at lower levels can kill you in one/two hits. As you may have noticed, better equipment starts appearing at later levels (this is also strongly influenced by your luck attribtute - for instance, I found dwarven equipment in Privateer's Hold, which is pretty much the best material you can find at lvl 1, with a char which had something like 85-90 luck). However, unlike Oblivon scaling, even at higher levels iron/steel equipment is much more common than adamantium/daedric/etc. Also, regarding armor, anything silver or orcish is VERY rare and if you find yourself with a full set of orcish armor you've really got too much time on your hands.
Is there anything static about the main quest?
Random monsters are scaled, but there are also lots of fixed-level monsters which don't scale (for instance, Direnni Tower is full of vampires both at level 1 and at level 20, whereas Nulfaga's Castle has battlemages/spellsword (with the occasional zombie) at lower levels and liches/ancient liches when you're stronger.
Could you tackle the main quest at level 1? 5?
There are level requirements for most quests in the Main Quest: for instance, although you get the letter from Brisienna as soon as you get out of Privateer's Hold, you won't receive the letters which allow you to start one of the first three quests (Nulfaga, Llotung's brother and the other one I can't recall right now) until you're level 3. Same goes for most of the later tasks - IIRC, you won't get the quest to access the Mantella Crux till you're 12-13th level (unless you're using the ship exploit, obviously).
How is Arena scaled, if at all?
Can't remember exactly, but I'm pretty sure the monsters aren't scaled at all, at least in Main Quest dungeons.