ColinJ,
firstly, there should be no issue of low sugar.
It is a while since I made yoghurt, but I checked on
http://www.makeyourownyogurt.com/make-yogurt/what-you-need
and as I thought all you need is milk and a yoghurt culture - which in turn is just a portion of live youghurt.
If you want to go low fat, then use low fat milk.
Secondly, don't be mislead by commercial youghurts.
As far as I know a good fresh home made youghurt is generally creamy but not set.
Commercial youghurts often have extra stuff in them to make them set because that is the way they are marketed.
However unless you particularly want it to be set, it is just as healthy and tastes just the same if it is like single or double cream.
Not all commercial yoghurts are set - you can buy runny ones as well.
So, in summary, I don't think your wife is doing anything wrong with the yoghurt maker - sounds fine to me.
You don't need anything but milk and a culture and once you have made your first batch then the last bit of your first batch is your culture for the second batch.
Cheers
LGC