If you are even the least bit handy with tools, some 4x4"s a skill saw and some lag bolts and you will be good to go. Barring that, I would watch the the local paper for a good deal on something. I know many people advise against them, but a decent dresser can suffice especially when you are not filling the tank all the way.
As for plastic plants, I have always been told to avoid them as turtles are slightly dumber than rocks and tend to try and eat them. While they are smaller, Non edible decorations might make the tank more enjoyable for you and them. Be careful of anything where the only exit is down though.
Lastly, I would try and keep at least a screen cover on the tank... you would be amazed how fast the can grow then given proper space and food. And you WILL be amazed at what a turtle can climb.
I would also like to offer you some serious kudos. Many people have taken on turtles with no idea what they have gotten in to, You however have asked questions, learned, and upgraded where needed.
And personally, and for your turtles, I would like to thank you for that.
The kitten, and some turtles that were not much bigger than yours not that long ago..
