First, give this some thought. Most 'end-fed' antennas require a ground system to work well. That ground system is basically the 'other half' of the antenna, and the better that ground system the better the possible performance of the antenna.
1: is thicker wire better than thinner wire for receive and broadcast? min guage?
The wire should be large enough to hold up it's own weight and then a little bit more just for general prevention of it falling down. After that, the 'size' of the wire makes no appreciable difference.
2: what is the minimum length I should consider?
The length is one of those things that varies quite a bit. A general 'rule of thumb' is a quarter wave length at whatever the lowest frequency of use will be. Longer is better till you run out of room to hang the thing, or wire, whichever comes first?
3: is aluminum better than copper wire or vice versa?
There are differences in the 'conductance' of different metals. Those differences make no differences at HF radio frequencies. Use whatever you happen to have the most of, or can find for a reasonable price. If that wire happens to be insulated, it'll work just fine. Same for bare wire. It just doesn't matter. One thing about that is that aluminum is kind'a hard to solder to, so up to you.
4:would doubling the wire help or hurt AKA a U turn at the end all the way back to point of origin?
Maybe, but not really. 'Folding' wire, making a 'folded' antenna, does affect the impedance of that antenna. Using twice the amount of wire in such a folded antenna doesn't increase the amount of signal radiated or received. It also depends on how much distance is between the folded conductor. Small distance is the equivalent of a single wire. A large distance makes the thing into a sort of 'loop'ish thing which can change it's radiation characteristics. That can be either good or bad, depends on the particular shape and what your purpose is.
5: can I make my long line in a L shape horizontally to eliminate the semi-directional nature?
That's a good question, and I won't predict it helping or not. It'll be sort of directional no matter what you do unless you stick it straight up. The reason most people use an 'L' antenna is lack of room to make it straight. But they do work, so if you want to try it, do so. But try it another way too so you'll have something to comparir it to.
Have fun.
- 'Doc
One last thing. The impedance of a 'long wire' antenna can be almost anything, so a way of matching that impedance is a very good idea. A tuner is one way of doing that.