Initially they would be fine and terrapins are fairly easy to keep, but do require a reasonable amount of maintenance (at least weekly water changes and bi-weekly tank cleaning).
I have a False Map turtle and a Mississippi Map and they're great!
Remember that you'll need a UVB bulb in the tank as well as an incandescent basking lamp. The filter in that tank should be ok initially as long as you are prepared to scoop out any waste with a net.
Also, terrapins generally grow to be quite big so you will need to upgrade your tank to a much larger tank (approx. 10 gallons per inch of turtle) in a couple of years assuming you buy juvenile turtles.
Have a look at www.turtletimes.com and www.turtleforum.com for loads of info and links.