I'm doing a playthrough where I'm only dungeon-diving and freelancing before moving onto the bigger quests (guild quests, main quests, civil war, daedric, etc). The goal is to level up, get good equipment and perks, then tackle the big questlines.

What would you say is the perfect level for this? Meaning the best/useful perks, good leveled items/rewards, and more. I wouldn't know cause I never made it past level 30, and only made it past level 20 once. Also, I'm a level 6 warrior with emphasis on block/one-handed/heavy armor.

I assume Daedric is the best armor (I ain't gonna level smithing, so forget about dragon armor), so when does daedric equipment start to spawn?

Also, I payed 1000g for a horse at Whiterun, but he likes to fight... talk about spirit... can he die, and if so will he respawn somewhere? Also, assuming he gets left behind - what then?

I'm not too sure about your other questions but with this question, yes, your horse can die, quite easily actually. If you leave him somewhere, he will either fast travel with you or go back to his original stable to wait for you. I forget the rules governing this, but I think it might be if you fast travel in the same hold, he will follow you.

I suggest you get used to the idea that this horse ain't gonna last you forever, or even the rest of the month. They like to get into trouble and don't take falls very well. Or maybe I'm just a terrible horseman.

Well, I know you can get free horses from the Riften Stables after beating the [censored] out of the owner.

But I'm doing a no fast-travel and no wagon-riding playthrough.

I rode to that fort near Whiterun, and much to my surprise the horse killed more bandits than I did, and did it without trouble.

Yeah, horses will occasionally fight off enemies impressively. It's hit and miss with them. I think there are other stables where you can ride a horse for free if you do them a favor, the thing is those horses won't wait for you when you dismount. Also if you pick a side in the Civil War, you can usually take the horses from a military camp for free.

Meh, I'll try to keep my current horse alive as best as possible.