Hi.
No, we paid the security deposit into the SafeDeposit trust (or, rather, to the landlord who sent it there). They took it and the first month's rent at the same time as we signed the formal lease agreement. They also allowed me to pay everything by credit card, since I couldn't get a bank account set up until I had proof of residence.
I did deal with a few letting agents who were trying to not use SafeDeposit - and they just hit me as sleazy. So I didn't pursue properties they were representing.
I had only a couple of agents discuss 6 months in advance, but the majority did not. Our current landlords are a large, well-established organization, which is a bit different from renting from someone who only owns one or a few properties. I had the advantage of having an Irish passport, and that was good enough for them. My daughter is not on the lease, although we have written permission for her to be living in the flat. We did have one landlord turn us down because she was going to be a student - they said their insurance prohibited renting to students.
I did send our current landlord quite a bit of financial information up front and I think it helped.
Hang in there!