Your gray water volume from your sink, if significantly different (greater) from your toilet's production volume, will limit the utility of installing a composting toilet. These are a nice alternative to a pit toilet system, but still expensive. Also, they have to be sited within the building to allow access to the solids pile they produce from their decomposition process.
Do you have a well? Check your county's code specifications on well and tile field seperation distance. The expense of a septic tank system is largely driven by the size of the tile field needed to steer the filtered water coming off the septic tank where bacterial decomposition is occurring.
You will need to conduct a percolation test to determine the rate at which water will move off your tile field pipe array area in saturation conditons. This will be used to determine how large your tile field needs to be, based on the soil type(s) at your site. Basically, a tile field is a French Drain application, consisting of a series of pipes surrounded by gravel. The pipe runs have holes off the pipe's invert that allows the water to drain and be consquently filtered as it percolates through the gravel and soil. Your local building specifications should give you guidance on how much course gravel underlayment you need, as well as how thick the pea gravel layer around and over the tile field should be. When you are done, measure from a couple of permanent reference points to the cover on the septic tank. Record these measurements on a sheet and put it someplace for use when it is time to pump the tank.
Slowly over time the solids left from bacterial decomposition will accumulate in your tank, rising eventually into the bottom of the T line installed at the tank outlfow. The pipe invert elevation and inflow pipe elevation difference should be in your construction code documents. One is slightly higher than the other. For a thousand gallon tank in a seldom used building, a bi-annual pump-out schedule is still likely too frequent, but it will give you a snapshot of how fast solids are accumulating over the first few years of use. Should the solids accumulate enough to start to seep out into your tile field, they will plug the drain holes and the whole tile field will need to be replaced. There are T filters that can be installed in the stand pipe at the tank outlet that will block solid outflow and preserve the tile field's function, but you need to still determine a tank pumping interval to keep the system funcitoning.
sounds like a heck of a shop though... Must be the home of a new batting cage and pitching mound....
