What about something like this:
http://www.plastic-mart.com/category.aspx?cat=41 The bigger ones are close to 700 gallons.
Since you want to use a big pond you are going to have a lot of water. A lot of mucky water. You may get away with emptying it only once a week, but it's gonna be pretty manky by the end of that week.
Filtering is very tricky. Ducks will put a LOT of dirt, sand and plant material into the pond. That will make life very very hard for any kind of filter. You can lessen it if the ducks are unable to reach the ground while swimming in the pond and vice versa. But you will have to raise it up quite a lot to achieve that.
As I mentioned, I use a Netafim greywater irrigation filter for my duck pond water before it gets pumped onto the garden (has to be filtered as it goes through dripperline). Every morning I put about 40 gallons of water through it and usually the filter medium has to be taken out twice each morning and hosed clean. The filtered water is definitely
not suitable to be put back into the pond - the filtration just takes the lumpy bits out before it goes onto the garden. A biological filter system would not need the same kind of attention but, IMO, it'd still get eventually clogged up with muck (if your ducks have access to dirt, sand or plants). Still, I know others more intrepid than myself have made it work.
As I see it, your options are:
1. Empty the pond at least once a week and distribute the dirty water to the garden and then refill with fresh water. Distribute the dirty water via a drain pipe to a sump that has a sump pump in it and from there pump it onto the garden. Or distribute the dirty water straight out onto the lawn/ground via a drain pipe. Or build a bog garden next to the duck pond and drain the water into that. The ducks will like dabbling in the bog garden (this is the approach I use with my duckling pen).
2. Learn as much as you can about biological or skippy filters and build the biggest one that your time, wallet and strength can manage. This is your only chance of being able to re-use water
3. Use a much much much smaller pond - which will dramatically reduce the size of the problem.
You also asked about drains. It is not hard to put a drain in. You will need some basic plumbing supplies and a hole saw. If it's too techy you can hire a plumber to do it. It'd take them half an hour to put in a drain at the bottom of the pond, a 90 degree elbow and outlet with a valve you can open for draining. That'd need to be done before you put the pond in the ground. You'd need to dig a small trench to accommodate the pipe coming out of the pond. Make the pipe diamater no less than 2 inches (to avoid clogging).