Hi there
I have a 4 year old daughter and a 3 year old step son who we have here most weekends, so there is a 16 month gap. I am currently 19 weeks pregnant with number 3, so I don't know how the age gap will work out either, but I have also been thinking about it a lot!
I think the good points will be that the older ones are getting more independent now, so can do things like get dressed and put their shoes on. They are also very excited about the baby, and seem keen to 'help out' when it is born. It also means that the older two will both be at preschool/school when it is born, so I will get one to one time with the baby to go to groups etc.
On the downside, the older two are very close, they have an identical sense of humour at the moment, and can regulary be heard laughing hysterically to each other for ages! They like the same kinds of toys, are physically very similar for things like riding bikes, running speed etc, so they get on really well. When the baby is their age, they will be 7 and 8, and will probably not find poo and wee funny anymore, and will not be quite on the same wavelength as the baby will be by then. I think they will just have a different relationship that the two older ones, they will probably be more protective of it and hopefully they will still find they have things in common.
This is the first baby that my partner and I have had together, the other two are from our first marriages, so we haven't ruled out having a 4th one in the future - but if we did i think we'd make it quite close in age to this one so that we have two pairs, (the older two and possibly younger two) both similar in age if you see what I mean!