GAH.
You claim you searched yet you clearly didn't read the "how to search this board" stickies either in the Q&A or here or you'd know to use google.
Read the PP through the end of the Novice chapter and read the Lift descriptions in SS... the rest of the literature you can read at your leisure.
It doesn't matter in the beginning but you should get some chalk.
You absolutely should not do GOMAD. Just stick with your diet for now as you've prob still got a good 80lbs of fat to lose. Lifting on no carbs does suck though so you may have to add some post workout carbs a few weeks into this, but considering that you're someone who comes from the very heavy side i think you should prioritize fat loss above all else even if it interferes with optimum progression on the strength. The 100lbs you lost so far is solid, but you still got a ways to go and it just gets harder as you get closer, so i'd be concerned about derailing that.
Read the "A clarification" article. http://startingstrength.com/index.ph..._clarification
This board in general tends to bias towards people being fatter rather than leaner. So that you don't have your perceptions messed with too much, i suggest that you get yourself a DEXA body composition scan (if there is a provider in your area). Basically, unless you're over 6' tall and/or squatting more than 3 plates you prob. don't need to be over 200lbs. (<- that's not official SS stuff just my opinion). How tall are you?
I still bench without collars though i do it with safeties. Obviously the most important thing is to put the safeties so that the bar doesn't touch your throat but you can still solidly touch your chest with it.
On the lower back thing... hard to say. There's a bunch of threads in the Recovery forum about back issues. Read through them. Draw your own conclusions. It may still be worth it to go to a doctor and get an MRI just to check that there's no major deformities.
Also, chill w/ the font sizes!
Good luck w/ the whole thing. Since you once had a 300lbs bench i suspect you'll be able to get pretty strong pretty fast on the other stuff too. Besides, your legs should already be pretty strong from carrying you around for the past few years (body builders would no doubt kill for your calves). I'll just repeat my warning from above... just because you're likely to be quite strong, don't let that distract you from your fat loss goals. 200lbs @18%bf w/ a 4 plate squat is much more impressive (imo) than 250lbs at 30+% bf with the same lifts. Lifting seems to make it easy to get "trapped" with a 250lbs powerlifter body.