I would go bigger if you have the space and know you will be a keen fish keeper for a while. Concrete tanks are great and easy to get much bigger for not much more.
To answer your filtration question, look at an internal filter behind a false back wall with multiple chambers. I would use bottom drains and sloped corners to have a self cleaning tank.
To answer your filtration question, look at an internal filter behind a false back wall with multiple chambers. I would use bottom drains and sloped corners to have a self cleaning tank.