Stuff work, they might not exactly be that surprised if you have another (although I did have to half joke that there wouldn't be a third), and if you've gone/go back then at least they know you are serious about working and not just taking them for a ride.
Kids are costly things, but that's no reason not to have them, I mean who has enough money these days? You'll always find a way. Sit down regularly with your partner and go through budgets and decide where the money goes and review it so there's no hidden costs and one of you doesnt feel like the other is spending it on unnecessary things.
I've found 2 no more exhausting than one to be honest. The 1st (DS) was hard work as a baby and the second (DD) is a piece of cake to look after in comparison (pure luck), DS is used to me feeding the baby and giving her some attention and is good if I explain why we have to do things a certain way because of her. (I got pregnant 2nd time when he was 13months so potentially a similar age gap to you)
No advice about being at work FT with 2....but this is due next month, I'm dreading it as I've really enjoyed maternity leave this time but needs must. Maybe the only good thing is the house stays tidy while they're at nursery/minders? and mine will both be in the same nursery so at least they see each other.
I thought about how a 2nd would impact on the 1st, but when DD laughed for the first time at DS it was impossibly cute and I hope by being close in age that they are close forever, me and my sister certainly are.
There's no right or wrong, but deep down in the depths of your heart do you see 2 kids in the future? If so go for it, but if that thought just stresses you out, stick to one and enjoy the little bit of extra sleep.