From what I have read and heard mollies can be converted over to full SW
but they only live about 9 months and only one person actually had them breed. You would have to get rid of the plants and other fish otos and cories don't do well in SW
maybe a day or two.
To convert you would need aragonite sand and live rock figure about a pound per gallon. You will want to change out the light bulbs as well to a 10, 14 or 20k bulbs. Use RO
water from the start to help stop algae. A good filter something like a 70 or 110 aquaclear and a couple of powerheads for added water flow. You probably have a heater already.
Things that I feel is very important it a good liquid test kit, and a refractometer. Take things slow only bad things happen fast in SW
and in a 20 gallon they can happen very very fast.
For the cost it will vary if you can get things used. For live rock I get mine off craigslist usually for between 2 and 3 per pound compared with 7 to 20 in LFS
. Sand is about a dollar a pound.