My 3 YO has bonded really well to us, but that is still forming (she still gets confused on visits with previous FM/FD). I think she's not typical though. She was an extremely laid back toddler even at the previous home
(who had her since 11 months). I would say she's even slightly ahead developmentally - which is suprising considering her first 11 months.
My situation is a little different - she is the youngest, and her older sibs baby her quite a bit. It may be different for the oldest/only.
My 4 YO is a holy terror toddler most of the time, but she has attachment issues and developmental delays that exacerbate the problems.
I would say go for it, but then again I am coming from a large family mentality and don't really have the mothering desire for an infant - I do much better with children I can talk to (though that presents its own problems
)