Hi!
I currently have DD#1, just turned 3, and DD#2, 16 months. We're expecting #3 the 1st week of Dec. DH and I both work, plus I take classes on the side, when I can squeeze it in. When we had #2, our first was 20 months. We had both in diapers for over a year. My brother, around the same time, had his two girls about four years apart, so I can tell about having them closer and having them a little farther apart.
I think we hit the timing with #2 very well: #1 was too young to really be jealous or resent having a sibling. We hadn't started potty training, yet, either - sometimes, if the child is in the middle of training or just trained, you can expect a little regression from the stress of adjusting to a new person in the family. My brother dealt with that big time! Things can be a bit chaotic, and we are admittedly sleep deprived, but it's a good kind of crazy, if you know what I mean. My girls are so good to each other - they haven't really ever known life without the other one being there.
On a personal note, my feeling was that if we were going to have diapers and bottles, I'd rather have it all in one long blast than several shorter periods spaced out. It can be a lot of work, but once you're used to it, it's not so bad (kind of like a Cathe workout, right?) I think that if I ever got a break from it, I would find it more difficult to get back into the routine, if that makes sense.
I am not going to tell you that I have everything perfectly organized and that I get regular workouts! I struggle to find time for everything, just like we all do, I'm sure. I try to take it a day or two at a time and work out as much as I can, when I can. It's more of a long range effort - I know there will be a day when I will be back in the groove and have my daily plan, just maybe not today. In the meantime, I try to hit a balance. If I can't work out, I try to eat a little cleaner and look for ways to squeeze some exercise in: park farther from the store, take the steps, do squats and lunges while cooking/cleaning, etc.
I wish you luck in your decision. If you take the next step, I think you'll find it's a lot of work, but really worth it. When you have more than one child, the interaction between them is such a joy to watch - there's nothing that melts your heart as much as when one child hugs and kisses the other one, or shares something with them...