There are so many things to consider. I'm sure you are trying to weigh all of them!
No, you are not weird. You have a lot to think about. I think it's very adult and responsible of you to consider everything before jumping in.
I can't really have a completely objective view of this, because I have 4 of my own children and wouldn't have it any other way. However, here are some things I think I would be thinking about......
We had our own "surprise" 7 years after we thought we were done. My older kids are involved in a lot of activities and trying to coordinate naps, bedtimes, eating on the go, and a now mobile 10 month old......is exhausting. The kids have expressed interest in going to a few movies in the theater - which we can't do because of the baby. They don't seem too upset about it, but I feel guilty. We had to move the older boys to a different room (in the basement) to accomodate the baby. It's a lot of stuff that I feel bad about. Mostly, though, the carting around of the baby to all the older kids things is the most challenging thing for me. If you have family/good friends in town that could take the baby so that you could have one-on-one time with the older two, that would be very helpful. Would your husband be able to help out a lot with this kind of stuff? Maybe your older kids don't have a whole lot of the extras, but if they don't now, it could be a different story in a couple of years;)
How do the 10 and 12 year olds feel? I know it's ultimately not their decision, but they will be affected by a new sibling. My kids were very happy with their little sister. I often ask them if she's worth it - the sacrifices we've all had to make. They always say yes :) However, if you do decide to have another baby, the older kids should be prepared as much as possible. "We won't be able to ......as much anymore" or "A new baby means a little more help around the house" or "Legos, pencils, small items will have to be off the floor" (that last one is a biggie in my house!!! LOL)
As far as the travel part, I hear ya. We've resoved to the fact that we won't have any "us" time for quite a while. Then again, what's a couple more years, right?
My husband and I couldn't do this, because we are INCREDIBLY fertile people, but you could do a wait- and- see apporach. Give yourselves a time frame and say that if you don't get pregnant in the next 4-6 months, for example only, it wasn't meant to be? Then be ok with whatever the results are and be prepared for the outcome either way. In my experience with friends who have done this, though, there's usually an outcome that is preferred.
I'm not trying to sway you either way, because it's absolutely your decision. Just know that you are not weird for not automatically wanting your own biological child. If you do decide to have a baby, I was 37 when I had my fourth, and women are having babies later and later in life with beautiful results. However, they will inform you of the risks involved and I eventually no longer got offended with being called "advanced maternal age" ALL the time!! :)
Good luck!