Alan -
Just about any general-purpose wine yeast should be fine. I'd wait
with the sugar until you can measure the S.G., and adjust as necessary
to the low end of the winemaking range (say, 1.080 or so) since beets
tend be high in sugar, but it won't register initially as it is bound
up in the beetroot flesh. Not sure whether pectic enzyme is necessary,
but it doesn't hurt - I'd add some, just on principle. I wouldn't plan
on adding much water later on - that just dilutes the wine. Add what
you need up front, adjust the sugar and acid as needed. Wait until
fermentation is nearly done (S.G. down to 1.010 or so), then rack into
gallon jugs. At that point, fermentation should be slow enough that
you can fill them pretty nearly to the top without much risk of
overflow.

There is really no very good way of stopping an active fermentation,
and I don't think you need to. It will stop when the food source
(sugar) runs out. Let it ferment dry, wait for it to clear, then add
sorbate and sweeten as desired at the end of the process.

Doug