Co-sleeping properly with proper precautions is very safe. Like you've mentioned, accidentally falling asleep on the sofa is less safe, and what gives co sleeping a bad name, I think.
One little point - it's not SIDS if they die during co sleeping. Then it's suffocation, or something else. SIDS is when there is a lack of cause.
I read Three in a Bed - a fantastic book about the benefits of co-sleeping: not just the bond it creates, the ease of breastfeeding, but also about how your breath on their faces as you both sleep induces them to take a breath as sometimes a kind of sleep apnea can come into play, and they don't take a breath for ages. Your breath out will encourage them to take a breath in.
Practically, we did several things:
Get a cheap Ikea cot, Gulliver, because it has two heights and the side comes off, and it fit perfectly against my side of the bed.
Always had him on my side, not between me and DH.
Always above the duvet with the duvet around my waist - DS was born late October so I wore a vest I could pull up easily and a cardigan. He either was in a sleeping bag or had his own cover - cellular blankets - over him and tucked around his waist. I got the Gro Company sleepsuits with padded arms to keep him warm.
I absolutely love it but was nervous the first night. He actually fell asleep on the changing mat which was on the bed so I just covered him up and cuddled up to him on that, and he slept on it the entire night! He'll be a year old on Sunday and we still co-sleep. Love, love, love it. He's still feeding one or two times a night and it has definitely reduced my tiredness - I just roll him over to me, latch him on, fall back asleep.
www.amazon.co.uk/Three-Bed-Benefits-Sleeping-Your/dp/0747565759