Honestly, you can never know. It's such a personal decision to have another baby. I can tell you my experience and tell you what I'm picking up as an outside observer.
My children are 20 months apart by accident (they are now 5 and 3 1/2). My birth control apparently wasn't strong enough! I can tell you that there have been both harder things and easier things about having them so close in age. They do have two completely different personalities. My daughter had colic, my son didn't. My daughter is fiercely independent, my son is more laid back. They play together, they fight together. I don't think there's a certain age gap where this doesn't occur, because I believe it's more in the personality of the children than in the age difference. My daughter hated the stroller, wouldn't sleep anywhere but on me or home, and my son would pass out anywhere and loved the walks in the stroller. So just know that just because you had certain experiences with your son still means there's a 50/50 chance for anything.
One good thing was that we didn't have to buy anything but clothes. I was just barely out of the diaper stage when we had our son (my daughter was beginning to potty-train when he was born), so there wasn't any huge change there. But we already had a crib, an exersaucer, and a bunch of other "baby stuff." We were prepared! My friends threw me a baby shower and all I got was clothes, because that's all I needed! I wasn't having to switch my mindset.
Dad was also pretty uninvolved until the babies were older... not because he was a bad Dad but because he just didn't know what to do with a baby that wasn't interested in him. I still don't get a lot of help with the housework, so I'm thinking it's just a man thing! As soon as my kids were old enough to show an interest in him as a playmate, he got more involved.
I did want to tell you that if you are lonely, the best thing you can do is join a play group or Mommy group like MOPS (you can visit MOPS.org to find one in your area). It's important that you have that Mommy time to connect with adults and have a true adult conversation every once in a while. Then you have real friends who you can vent to, ask Mommy questions to, and just take a break with. These kinds of groups were my lifesaver when my kids were little.
But, again, when to have your next baby is a very personal decision. I've known women who swear by having them 2 years apart or less, I've known that swore by 3 years exactly, I've known women who swore 5 years, 8 years, etc. It's very much just about how you feel.
Good luck to you and let us know what you decide!