Repurposing furniture is a great way to save money. As far as the height of what you want to use for cutting and sewing, that is strictly a personal preference combined with ergonomics. There are several ergonomic sites for sewers online. Many pieces of furniture can be lowered by shortening the feet or legs, or raised with the use of locking wheels.
Also remember the height of your sewing machine to the bed of the machine. Some of my friends did not make their sewing tables with recessed areas for their machines. Instead they bought larger fitted acrylic table tops and feet to keep the acryllic table top the correct height.
If you get a complete room to designate as your sewing room consider other items to incorporate in your plans: fabric storage, tools, rulers, pattern storage, possibly a TV or computer nearby.
I set my studio up so that I could watch the TV in the living room by just rolling out the sewing cabinet. I rarely put it back as most of my friends know I quilt/sew.