45# is a good weight for a
longbow. (English
longbow... ELB)
Not sure what you mean by "equivalent draw weight" obviously 42# is equivalent to 42# but if you are looking for equivalent performance you would need to go up a tad.
I'd expect longbows to be pretty much quoted in increments of 5# it's difficult to measure/build to much better accuracy than that.
the main difference is that a longbow won't get held at full draw for ages so you can prob' go up a bit in draw weight. They will generally be quoted at 28" draw too, also note draw length on an ELB is measured to the back of the bow.
Look for a good reputable bowyer... note, that doesn't necessarily mean a mass producer, there are plenty of smaller producers and good amateur bowyers. I'd always suggest visiting the bowyer if possible to try before you buy and I'd warn strongly against buying from E-Bay.
Del
PS, also depends on what discipline you are shooting. Field would have you shooting fewer arrows so maybe you could stand a little more poundage.