Generally it's your passport, a certificate of residence and possibly your lease. Go to the branch and enquire. It seems there's two possibilities at Emquartier so try both
I don't know, I never use agents, I just know from a number of reports relating to opening bank accounts that Thai Visa Centre will only open a bank account if you're also obtaining a visa through them. I don't know about any others
You'll probably be questioned by immigration so have a damn good story as to why you need to stay in the country for long periods as a tourist. Your best bet might be to return to your home country and get a METV. This will give you a potential nine months in the country and will not set off so many alarm bells with immigration. However, if you have extensive recent history you could well be denied this visa
What you describe is fine. I think the "dress appropriately" advice is geared more towards those who wear flip-flops, beach shorts, and T-Shirts which say "bad boys go to Pattaya"
If you're returning to your home country each year, probably best to get a Multi-Entry Tourist Visa. This can give you a potential nine months in Thailand with two strategic border bounces.