On health - the reports I say said "likely carcinogenic" but they are far from proving it. They just like to think it is 'cause it is effective. One of the reports indicated the surfactant was more dangerous that the Glyphosate.
Application - I've been reading that anything less than 41% is ineffective.
41% is the concentrate, not what you'd actually apply. RoundUp is a slow kill, it takes 10 - 15 days for full kill and a few plants take a little longer. Mixing it stronger will show a more immediate burn down but, it doesn't actually kill the plant. I use a handheld sprayer for my RoundUp and mix it at 2oz 41% per gallon of water, which comes out to about 2oz per 1,000 sq/ft. That mix won't show any plant stress for almost a week but once the plants do start to curl at the leaves, they burn down quickly.
With most herbicides, a little patience is required to let the chemistry work if you want a complete kill. There are some that will burn a plant down in 24 - 48 hours but, they don't kill the root system so the plant will come right back in a week or two.
As was already mentioned, RoundUp metabolizes out as soon as it makes ground contact so it will do nothing to prevent regrowth. Since I'm intensely lazy, I like to use a pre-emergent in my beds once everything I want to grow has germinated. I have a license so I use Snapshot. Preen is also pretty good and can be bought at any home store, it just doesn't last as long. Depending on rain and humidity, I can get by with 2 applications of Snapshot a year. With Preen, you'd probably need 4 - 5.
If you decide you want to add plants to the garden after putting down the pre, you'll need to start them in post and transplant them once they're growing well.
