I have an older dd and younger ds, 2yrs and 4 months apart. There are times they love each other to bits and play well together, and other times when they fight like crazy (and drive US crazy!) I have to say though, that the best thing about having 2 kids is watching them grow up together and seeing their relationship strengthen - it's priceless.
Firstly, don’t make the mistake of telling a toddler you are having a baby to give them a friend – you are having a baby because you are building a family. A newborn baby cannot be a friend anyway – they are no fun for an older child at all! All they do is cry, sleep and feed – and seemingly take all of mum and dad’s time!
Think of it this way, siblings are probably the only ones in the household who didn’t choose to live together. No matter how carefully and sensitively you introduce the older child to the new baby, or how fair and equal you try to be as they grow up, you will never eliminate all sibling rivalry.
Before ds was born, we went thru dd's baby photos and talked about how she used to be very small and couldn't talk or walk or do any of the things she could do now, and needed lots of looking after. We made a big deal about being a Big Sister, tho not too much cos we didn't want her to feel stressed about some unknown big new responsibilies just yet!
She was pretty good when her new brother came home, overall. There was the usual attention-seeking behaviour esp durung feeding times when she knew I was unable to stop her easily. I think she was more curious than anything else, and someone told us not to be too overprotective of the new baby or she would resent him. So we held our tongues and clenched our jaws as she peered at him and prodded him a bit, and let her hold him across her lap (she was sitting on the floor) even though he screamed the whole time...! Another thing we took advice on was to give dd a gift from baby (which I wrapped up and hid in my hospital bag for her first visit), and she made him a card. And I had a few pressies tucked away for those times when baby got a gift in the mail or from a visitor but big sister missed out, plus she got to help open them.
This has gotten very long...woops! I had mild pnd after my first too. It's not easy, but I found I was so much more relaxed with my second baby, and he seemed to be a more relaxed baby too! You just don't let so many things get too you, less "paranoid" and more confident that you do know what you are doing! Go for it!