1) What ratio of compost to soil should I use when filling the bed?

Enough compost to make the soil crumble fairly easily when you have "crunched"
a handful of it together, then prodded the ball thus formed. Do this test a
couple of days after a good soaking rain (or watering). Too much clay
will make the ball of soil stick together; too much sand will cause the soil
ball to fall apart with only the tiniest of disturbances.

Sandier or more compost-rich soil is nice for growing root vegetables,
because they harvest more easily. Clay soils are good for keeping big
veggies (like tomato plants) in the ground in high winds. About equal
parts of clay, sand and small gravel, and organic matter is considered
ideal garden soil (and it's nice to dig in!). Very sandy soil requires
more frequent waterings in many cases; very heavy clay soils are easy to
mishandle and squish the airspace out of (roots need oxygen!).

2) I am rural and have well water. It isn't too hard, but has a lot of
sulfur in it. Should the sulfur cause me any worries, and if so is
there anything I should do about it?

Sulfur is a required element for plant growth; along with C, H, O, N, P, K,
Ca, Mg (these are the macronutrients; each macronutrient is present in the
dry matter of most plants at 1% or above) and Fe, Cl, Cu, Mn, Zn, Mb, B
(usually 1-100 ppm in plant tissues).

The only thing I'd check on with a high sulfur water is how acid the
water is; if it's in the pH 6-7.5 range, you're fine. If it's more
acid or more basic than that, extra compost will help ameliorate the
situation. If you know what formation your well is getting water from,
a call to your county extension agent can generally find out if there
are known problems with the water.

Oh, and you won't be growing low-sulfur onions like the Vidalia types
with that water.

Suggested reading: first few chapters of Rodales' Chemical Free Lawn and
Garden,which your public library may have or you can pick up cheaply. Well
written chapters on soil, water, plant nutrition that are easily read.