It seems as if it is actually more complicated than attachment or not attachment- but also in some ways simpler as it is mainly about practical points which can be broken down and dealt with separately.
"DH can share parental leave and it's how we split it, basically. I have this feeling DH would be an awesome SAHD and he wants to do it. But I'm having an emotional response to the idea of leaving a very young baby (2, 3, 4 months)."
This isn't actually about how your dc is parented: it's about who does that parenting. Your dh can still provide the same level of closeness; the child can still be attachment parented.
"Then the BF issue follows on since if I was at work it would be harder"
Harder but not impossible. You might be able to express some bottles, or even give some bottles of formula without disrupting breastfeeding once dc gets to a few months.
"there is potentially some medication I could take when not BF that isn't advised when BF which could make a significant difference to my life/mental health"
have been there and the thought was agony. In the end my GP pointed out to me that stumbling around in my wrongly-medicated zombie state I was actually a danger to him, and that even if nothing happened, having a mum who was there for him was more important than the milk. It was hard to accept at the time, but with hindsight I can see that all the antibodies in the world would not have been much good to him if I had dropped him on his head because I was so out of it. Of course this depends on how much you need the medicine, but if you are spaced out without it, there will be less attachment than if you are alert and happy with it.
"And then if I don't BF and I'm working what would be the point of co-sleeping, since DH could/would do night feeds."
This isn't something you have to decide once and for all; you could test various solutions and see what works.