DIY or hire out? Requires architectural plans, building permits, inspections, run utilities - electric, water, gas, sewer.
This is a huge project will realistically run you into the tens of thousands.
Some of the largest expenses will be to build a concrete slab or crawl space to support the above-ground structure, and installation of the stack for the toilet and waste water.
If you are looking for an option that requires less plumbing, look in to options for a composting toilet. Then all you need is to plumb a faucet and drain which is less complicated than a sewage tie in. Some of the newer composting toilets are exceptionally well designed. I prefer the European company Separette.