Hello, I don't have experience of the exact surgery you're looking at, but my eldest had OHS when he was eight days old, so I have some knowledge of how you may be feeling. Is her surgery to close her VSD only, or is she having the aortic valve repaired or replaced as well?
Bank on being in hospital for a good few days, probably a week or so. Don't worry about the scar immediately post surgery, as they do fade an awful lot - I know some people use bio-oil to reduce the visibility too. There will be some restrictions on physical activity while her sternum heals, but this shouldn't be a long term problem.
More generally, do share this with supportive friends or relatives as soon as you feel comfortable. It is a stressful time, and having trusted support is invaluable. Speak to other parents on the ward - they're all going through something similar. Be prepared that surgery day will be unpleasant; everyone advises that you leave the hospital as time drags ever so slightly less than if you're sitting with nothing to look at or to do. We spent the day of our son's surgery wandering round Leicester city centre in a bit of a daze, but it would have been even worse sat in the hospital.
In terms of specifics, try to speak to the cardiac liaison nurses at the hospital (which centre will you be at?). They're brilliant at explaining things in terms that are easy to understand, without over-simplifying things. And I can recommend the charity Tiny Tickers www.tinytickers.org/. While a lot of the information refers to babies, much is relevant across the ages.
I could write for pages an pages, but that covers the main things that spring to min right away, but please do feel free to ask anything else.