Welcome, I hope you know after your first you will be hooked
Barrel length depends on what you will be using the gun for and what range you will be shooting, I have a 16" and a 20" but I usually shoot 100 yds or less so I can't tell much difference in accuracy.
If you are planning on putting optics on your gun I would go ahead and get flip down sights so you only have to buy them once unless you are wanting to co-witness a holo type with them, then you can go either way.
trigger depends on your preference If you are buying a gun kit you can use the one that comes with it and upgrade later.
As for manufacturers you might try J & T, M & A, Model 1 sales or Del-Ton. I have a Model one kit gun and a J & T also, the J & T costs a little more but in my opinion it is well worth it, everyone says del-ton makes good stuff but they are really far behind from what I hear.
Best thing is to lurk around, look at the pics of guns posted and try to figure out what you really want, there are millions of possibilities.