Merino wool is a very soft yarn--lovely to work with and hand-washable, usually.
You don't need merino specifically for the sweater, any worsted-weight yarn will do. Then you work with your needle size to match your gauge to the pattern's gauge.
both have some lovely yarns at reasonable prices.
You can also go with a soft acrylic or acrylic blend like Plymouth Encore for a machine-washable sweater. Stay away from the cheap buck-a-pound acrylics, though. They don't make a soft sweater; and for the time you're investing in a sweater, it's soooo worth it to get a decent yarn.