I'm about to start a new playthrough of oblivion and I want to do this one as a stealth based character, I've done mages and warriors before but it wasn't until I did the thieves guild that I realized how much fun being stealthy can be. anyways I want this guy to be a freelance theif for as long as he can and make as much money as he can before he joins the thieves guild. before I start what are some good places to do some freelance thieving with really good payouts that also give a real challenge. on a earlier stealth build I robbed some of the castles at low levels but they were really easy and didn't have that great of loot. also with oblivions level scaling does the castles loot get better or is it set? one last thing, what level should I get my guy up to before starting the thieves guild if I want it to be a real challenge but not insanely difficult.

I'm not sure if you can fence stolen loot without the TG, can you?

Manheim Maulhand at the Inn of Ill Omen will buy stolen items, but you need to have a Journeyman Mercantile to sell him anything other than potions and ingredients, and he's only got 50 gold. Slim pickin's for the freelance burglar.

I have the pirate dlc so once I get a thousand gold the first thing I'm going to do is hire a fence to sell all of my stolen goods unless I have already joined the thieves guild. once I get up in levels he is going to be foolish enough try robbing the imperial palace and get caught and sent to jail, when he gets out he is going to join the thieves guild.

Well, then in that case.

Well, the Imperial Legion Commander's chest right beside his desk may not have the best loot on the very first go around, but any subsequent loot it respawns with is top notch. :tops:
AFAIK, the loot in castles stays the same/ is set. It may respawn, but with relatively the same stuff on the regular game.
I don't think the TG quest line matters so much as to which overall level you are at. It may depend more on how high your sneak skill is, and how good you are a lock-picking.