We've got the angelcare one and are glad we did - has saved so much going in and checking on DD, helping us all sleep better. I'd second everyhting angel1976 said about it - we also have a cot bed and only once ever had a false alarm (in 10 months) - DD was right up at the end of the cot, I think leaning on the end rather than the mattress), but she moves around a lot, often to the very ends and that it the only time it went off. It doesn also go off if we forget toswitch it off when we take DD out, but obviously usually holding an awake DD then so no panic - maybe just make sure you DH tells you if he is getting DD out.
We always have the sound set to only come on if there is a noise in the room, so can't hear her breathing - You can switch it on constantly I think, but I think you would have to have it ridiculously close, to do that, and even then i don't think it would always work - I can't always hear DD breathing if I am next to her! But the idea is you shouldn; need to hear her!
The parent unit is portable. As for becoming too relient - we don't use it with the travel cot when going to visit relatives as not a stable enough base, and I manage not to worry too much, although I think it sounds like you are more worried than I was anyway, so I suppose you may get quite relient.
Whoops - just remembered you said you were co-sleeping - never done this (partly because i thought it was a SIDs risk, although since coming on here I know a lot do it) so not sure how the monitor would work with that - a bed is a lot bigger than a cot, and obviously no use in the night because you would be breathing too!
It does sound like your anxirty is bordering on paranoia and affecting your life, so perhaps you should speak to someone about it. I can understand why - but be reassured that the SIDs risk drops significantly after 6 months, so you are getting there!