I ended up pregnant with my DS when my DD was only 3-4 months old. It was a total shocker (I wasn't taking my BC on time everyday) but in the end was a blessing. He was born a little early - 37 weeks - but otherwise was healthy. He needed no time in the incubator. At 4 months, he was diagnosed with a rare birth defect called "malrotation" but this was easily corrected with surgery. We think it had more to do with me not taking folic acid in the beginning because I had no idea I was pregnant than the closeness of the pregnancies though.
The beginning was pretty easy. The only things I had to worry about was my DD wanting to poke his eyes. She became kind of motherly to him until he started walking, then she became a bit jealous. The jealousy eased up a bit when she was able to play with him, though she still gets mad at him if he takes her toys. She's never hit him, she just takes her toy back and says "No!" He used to get upset by this but he sadly is so used to it now he just goes off by himself and plays with his own toys.
The hardest I always thought was the finances. The cost adds up pretty quick when you have two in diapers. You also have to think about the cost of formula and food. Feeding them is hard too unless you have two high chairs. While he was eating, his sister liked to pester him to give her some of his food. When DS wasn't in his high chair, he would try to take DD's food. They are getting better at sharing to where I don't have to use the high chair much anymore and as long as they each have a serving of the same thing and each have a drink, there is usually no squabbles anymore.
On a positive note, having two so close seemed to make them best friends. DD won't try anything new unless DS does it first (you'd think it'd be the other way around but he is more daring where she is more cautious). When we do things, we do it together so that way I don't have to teach one and then re-teach the same things to the other. I read to them both, I showed them how to brush their teeth together and even potty training was easier because I could teach them both at the same time instead of having to do it twice.
Really it is up to you and nobody can decide for you except for yourself and your OH. I'd say if you want them close and your first born is already almost 1, you've given your body plenty of time to recuperate from childbirth. I'd say there is nothing stopping you from being able to have another, if that is what you choose. It is just whether you can deal with 2 under 2. Like I said, the beginning is rough but in the end, to me, it was all worth it