Ronnie,
With how much of a mess turtles make when they eat, and poop and shed skin, you need a very powerful filter. You want to filll that tank up as high as you can without the turts being able to climb out and escape. don't listen to pet stores that say you only need some water. Turtles natural habitats are lakes and ponds and can be several feet deep so fill that sucker up. As for filtration you want a filter that has a water turn over rate of about 3x the water capacity of your tank. Meaning that since your tank is a 70 gallon, you want a filter that has a gph rating of ATLEAST 3x that amount so you want to look for a filter with a gph rating around 250-300.
I have a homemade sump filter i made out of a 10 gallon tank. You could even use that old 20 gallon tank for a new sump set up (20 gallon longs are great for sumps). You can find ideas for great sump setups over at
http://www.monsterfishkeepers.com . All you need is to buy media and a pump and you could spend as little as 60 dollars total. Or... you could go with the storebought approach and get a canister filter such as a Rena or Fluval for around $150-250. If you have any questions about making a sump filter let me know. It really isn't hard and you can make one with very little tools or know how. Canisters can also be easily made. I just made a 2 and a half foot canister filter out of some 4inch pvc pipe and a pond fountain pump. I wouldn't buy that filter you have posted there. Its not powerful enough.