Hi Mick
Our setup is like the "..more complicated.." one that BillGiles describes - there's a selector valve on the side of the basin unit in the heads that switches between 'Sea' and 'Tank'. For the tank side, a large bore pipe runs from the valve into (eventually) the top of the holding tank, which is bolted to the bulkhead in the starboard locker. The tank has a breather pipe that exits through the hull just under the toerail. Our tank has two outlets - one runs up to a deck fitting for pumpout and the other goes to an electric Sealand T-series pump, which exits through an underwater hull fitting. As far as I can see, all piping passes through existing holes in the bulkheads (B34/2001/2-cabin). The Sealand pump could suck sparrows off the sidewalk so I don't think there'll ever be a problem with it getting bunged up. However, there are non-return (duckbill) valves either side of the pump, which can invert if it's turned on whilst the seacock is closed (in my defense, it was a newly-purchased boat and an unlabelled switch). Requires a stripdown of the hoses to fix (and a stripdown and rinse-off for oneself afterwards!).
In truth, I would think ?3K+ would be about right for our setup but is a bit steep for your requirements.