I have a 22 month gap and its early days, but going ok. Both children were somewhere between planned and unplanned IYKWIM.
Main thing is try to get actual support after the birth, but don't accept offers of help from people who will undermine you (my MIL-spent 2nd pg repeating "how WILL you cope" like a mantra, every time we eg had out of date yoghurt in the fridge. Now goes on about how much we are neglecting poor older ds-eg the day after his sister was born his breakfast was TEN MINUTES LATE! ). For me, this support was worse than useless.
Re loving the new baby-I worried about this too. I didn't find it an issue, BUT if it is for you, there is help out there. I spent whole pg feeling guilty about ds, but in the end, it is probably easier to love a helpless nwborn than a toddler who has just learnt to say no, and its all fine.
You don't need a twin buggy, but it might help. We have used ours a lot, as ds still wants to be carried also, so sling is not practical.
Also-if you intend to bf then completely toddlerproof the house as your ds will spend a lot more time unsupervised than you would like. And a playpen (for the baby) is a good idea.
I would be very honest with your gp/midwife. there are support groups, practical help, etc that you can get if need be. obviously they will look out for pnd but they also need to look out for depression now.