OK, I will admit to not having read this entire thread because this is an issue I can end up getting irritated about. I can see both sides of the have another (or more) and the only child issue.
I myself am an only child. I am also a mother of 3.
I get so irritated when I hear statements that make it sound like you are doing your child an injustice by not having more than one. Personally, I am more than fine with being an only and have never felt slighted in any way by not having a sibling. My parents said I never once asked for a brother or sister. I was just fine with things the way they were. I am very very close to both of my parents. I have a better relationship with them than most of my friends with siblings had/have with their parents. So there is a give and take there. Having a sibling for your child doesn't guarantee them anything. There are siblings who are as close as can be and always have a wonderful and special relationship and there are siblings who do not speak at all and have nothing but resentment for one another. You can't know which situation you are going to get. My mother has 3 brothers and a sister. One of her brothers and her sister do not speak at all and haven't for years over an argument their kids got into over a matchbox car of all things. Another brother has distanced himself from all the siblings after their father died. My father has an older brother. They don't have any ill will between them but the last time they spoke was over 4 years ago. They get along fine when they are together but one lives in GA and the other in Colorado and they just aren't close so they don't make an effort to stay in touch. Having a sibling does not guarantee your child a best friend for life. The best thing you can do for your child is to love them unconditionally. Have another child if
you and your spouse want another child but don't have another just so your first won't be an only! Being an only is not the end of the world. My husband and I wanted more than one for US, not out of fear of our oldest being the only. We had all sorts of fertility problems as well as multiple miscarriages and after months of giving myself shots and doctors appointments every other day, blood work, ultrasounds, procedures and all that we said we'd try for another but we set limits on what we were willing to do. If it didn't come easier the 2nd time around, then our oldest would be our only. Fortunately it was easier and we got pregnant on our first treatment cycle with DS #2. DS #3 was a surprise baby. For years when I went in for my yearly exam my doctor would ask what our method of birth control was and my answer was always "infertility and prayer."

9.5 years without birth control and we have one surprise. LOL
All that said, if you want a 2nd child because you feel your family isn't complete yet, there are pros and cons to close child spacing and to further apart. My first 2 are 21 months apart and there is 4.5 years between the 2nd one and the 3rd one. Having them close you do get all the diapers and car seat time all over in one fell swoop but that first year is a real challenge. The oldest is not quite 2 when the baby is born so the "big sib" is still at an age where you have to watch them constantly so they aren't eating the dog food or wraping the cord from the mini blinds around their necks. THe baby obviously needs your full attention too. Once you get through that first year though, it's great. They are playmates and they are for the most part interested in the same things. When it's rainy out and you want to pop in DVD, you can find a movie that interests both of them rather than one wanting to watch Harry Potter and the other wanting Barney. If you wait untill your first is older then they are more independent so you don't have to worry about watching them like a hawk. They are also old enough to be helpful. Not talking forced babysitting but just with grabbing things for you, cleaning their own room, can be trusted not to cram legos into the baby's mouth while you go to pee...stuff like that. On the other hand, in my experience there is more sibling rivalry/adjustment to a new baby with them being older than there was with adding a 2nd when your first is very young. I don't think my oldest remembers being an only child (good and bad to that I guess) and he totally rolled with the punches when my 2nd was born. He had a much harder time adjusting to the change in Mom's time and attention when baby #3 was born. My first two are each other's best friend and worst enemy all rolled into one. They get along quite well most of the time. I ended up being glad I had them close together like that. Enough so that when I found out #3 was on his way, I was worried about the age gap. It's too soon to tell how he will fit in "closeness wise" with his brothers. They do adore him and he just lights up when he sees them so I have hope that they will all be close.
Is it easier with the 2nd. Yes and no. Yes in that you know what to expect and you aren't as tense. That helps but it's far from
easy. I firmly believe what so many parenting books etc. tell you...that having a 2nd isn't twice as much work as having 1, it's exponentially more work. I found the transition from 1 to 2 MUCH harder and more work than the transition from 2 to 3. It was like we already have controlled chaos, adding one more didn't make a huge difference.
How you handle everything...you just do. You do it because you have to. You pray that your next baby doesn't have colic. Sleep deprived. Yep. That never gets easier. OF course note that I have a 4 month old and am posting at 2:51 am because I am WIDE awake after dealing with him.

Fortunately for me I am a SAHM so I can nap tomorrow with him while the "big boys" are off at school.
Give it some time and though. The risks aren't huge at 35 so don't feel like you HAVE to beat that line. Don't rush into it untill you feel ready because it really is a lot more work with 2. Not that it isn't worth it, but it is a lot of work. If you and your husband feel your family is not complete, then have another but if you don't feel that way, don't worry about your daughter. Being an only is not a terrible thing.