We found out that the pill didn't work for me when I found out I was pregnant with my son (3 1/2 months into the pregnancy!) Our middle daughter and my son are 18 months apart. I was very nervous at first, but I love it now and wouldn't have it any other way. One thing I would strongly suggest is getting a double stroller. I use mine for everything. I didn't think I'd use it very much because my daughter is very independent and wants to do everything herself and hated riding in the stroller with a passion. Then she realized that the would get to ride in the stroller and then she wanted to ride too. We have a van, so it fits nicely in the back and it fit in our Corolla, but not very well (I would have to take off a sunshade and cupholder to get it in). This is also wonderful for taking them on walks. There are many options available depending on your needs; we got a Graco tandem stroller. You can put two infant carseats directly in them (or just one because your oldest will be in a convertible or front facing seat by then), they have cupholders for you as well as both children and a snack holder for one child. There is also a place for your keys/cell phone that closes up. The basket for putting stuff in on the bottom is also huge to help hold the extra stuff and diaper bag. I also heard that the double umbrella strollers are nice for when you need something smaller or for walks. I haven't tried them and someone told me that some of them are hard to maneuver and you won't be able to have as much storage. Another thing that I've found very helpful is a sling or front pack for your little one. It helps carry them around, especially when your oldest wants to be carried as well and also lets you carry them while you are doing dishes/cleaning, etc. if the oldest won't leave them alone (my daughter is still obsessed with him and her little maternal instincts came in).
Another thing you'll need to make sure is that you have a big enough diaper bag/extra diaper bags. You'll need diapers/Pull Ups/wipes for both children, extra clothes, snacks, etc. When my son was first born, I used the Eddie Bauer diaper bag I got at Target. I felt like I was packing for a weekend trip, but I always knew I had everything I needed for each child. I also like to have a couple extra littler diaper bags around so that if for some reason, one child is going somewhere else, they can have their own diaper bag (example: dinner with Daddy or Grandma and Grandpa). This helps a lot. I usually keep a diaper bag packed in our coat closet for each of them complete with diapers and extra clothes so that I can grab it when I need it. I usually leave the wipes out because they would get too dry when I would leave them in.
As far as a crib, it will depend on your 15 month old. My daughter was ready for a toddler bed before my son was born. I like because they are low to the ground and have side rails on the top part. They use a standard crib mattress and sheets. You can also buy sheet sets with blankets for them. What we did with our daughter/son was have him in the bassinet in our room for the first few months until he started sitting up in it and got too big. By that time, she had adjusted to the toddler bed and then when he was ready for the crib, she no longer needed it.
My daughter was out of the high chair before my son needed it. You will want to get a booster seat for your oldest.
Lastly, you might consider buying stock in diapers :) Just kidding...I can't wait until my middle one is completely potty trained :) We'll be saving so much on diapers.
Good luck...let me know if you need anything...I live pretty close to Denver.