Is Sam male or female? If he's a male, there's a good chance he's pretty close to his max size. If she's a female, she may still have several inches to go.
If you think Sam is at his/her max size, then you could get away with a 75 gallon for the rest of his life. If you think he's got some growing left, the 75 gallon will at least last for several years, especially if you build an above tank basking area.
My personal recommendation for a filter is the Fluval FX5. I use one in my 75 and 90 gallon tanks. It keeps the tank clean.
For a screen, you'll probably have to make one. You can use egg crate (aka light diffuser). It's cheap (usually about $12 per sheet), and easy to cut to fit.
You could also get creative and use an old oven rack for the top of the tank. I've got egg crate on one tank, and a fridge rack on the other
