Moved this from the classifieds to the discussion forums.
Stock tanks may provide you with affordable housing options. You can build a frame or skirt to hide the "ugliness" of the tank and decorate to your liking. Water changes may be better for you than filtration, but it depends on how you manage things and what you find practical. Heavy duty filtration is needed if frequent water changes are not feasible. There are some DIY plans out there for cheaper gravity and pump designs that work well, but I have not looked in ages. Use plants (protected from the turtles) to assist with filtration as well. Pothos, water hyacinth, and mangroves are among commonly used plants. Google all of this if no one provides anything more (and then use Google anyway). I rarely discuss care because it is a topic that burns me out, but these mentions should be able to get you started. Good luck!