Regardless of whether your child has downsyndrome, you will still be older parents to a child without additional needs. Both are equally hard work, both need just as much energy as the other. You need to have been thinking about these possibilities for a child without additional needs, not just for one with. So, it doesn't really matter whether they have additional needs or not - Children are a massive lifestyle change.
I have friends with children whom have downsyndrome - They are beautiful, loving, funny, full of character, sassy, and live a fantastic quality of life in mainstream schools where they are assisted to thrive. The LA assist them so the parents can have a break, they go on day trips, special parties etc...it's not as daunting as you might think it will be.
It's a massive shock, there are the added disabilities, all kinds of anxieties and chaotic thoughts that will come with this news. However, you don't have to let these thoughts control you.
Whatever choice you make will be the right one for you.
I do highly advise you look at the Facebook page "Fresh As Daisy" - It's the journey of a single Mum and her daughter Daisy. Daisy has Downsyndrome, but her Mum is also an additional needs therapist/speech&language. Daisy will have you crying-laughing.