redearslider.com

I have a 2 red ear slider turtle , I am planning on making an outdoor tank for them
http://www.homedepot.com/h_d1/N-5yc1vZ1 ... ogId=10053

would this be enough? Do I need to add some other accessorys with this?

It might be OK, though the size of the turtles and your location will also be a factor.

The plant shelves on those preformed ponds can really cut down on swimming room, especially if the pond isn't that big. Can you find a preformed pond without shelves? If not, you might consider a stock tank.

How big are your turtles (shell lengths without the curves)?

id say go stock tank, i could be wrong but you might be able to get a 300gallon stock tank for the price fo that pre formed pond

Not unless you find one used at a good price. I paid roughly $220 for my first 300-gallon stock tank new (TSC). And found a 2nd and 3rd one for $75 each, gently used in the classifieds.