The general contractor chose a Pedrollo Multitech for me. It provides sufficient pressure and flow. Clearly better than what comes from the drinking water tap. However, it is also a bit more expensive than 50€.
If there is mud inside that also stinks, it has to be removed. So either crawl in and scoop it out (only possible if empty) or use a dedicated dirty water pump and then stir up the dirt and pump it out. For this, you need a pump that sucks from the bottom and possibly constantly agitate the dirt with the fresh water hose.
Our newly built apartments that are now nearing completion are being reduced from €410,000 to €310,000. The developers are already this desperate if they haven't sold their residential park by the time of completion. They have recouped the interest well, although until a few years ago, such an apartment would have cost at most €250,000.