In the kindest way, I think you're overthinking it... this baby will just fit in. You will not remember a time when s/he wasn't there, and you won't be able to imagine your life without them. I have never, ever, had a single regret. I know that's me, and I am not you.
I think maybe you should have some counselling? This is really eating away at you isn't it? The love just expands, the way it did when you had your second and I bet you worried sick that you would be disadvantaging your oldest, or you wouldn't have enough love for 2, before you had your second? I know I did.
Just look at those 2 wonderful little people you and your DH have created. How beautiful will it be to have one more?
You have age on your side so much too. I was 7 years older than you having my 3rd. I was 34 having my first. That's the only thing I'd change, having my kids younger - but then I wouldn't have had the children I have.
Even if you wouldn't consider termination, maybe access counselling that way?
I never understand either why people make a fuss about cars etc - I managed to cram all of mine in the back of a Vauxhall Astra or a Ford Focus. Never had a people carrier - never wanted one.
I can see how conflicted you are and my heart goes out to you. Listen, it's early days, and even if you've meticulously planned a pregnancy, I think you go through a phase of "what have I done?!" when you get that +.
Best wishes x