I had 2" memory foam toppers and just went to 4". Made a huge difference. It feels,like a new mattress, not just a softer lousy mattress. Don't get anything less than 4". Easy to cut to size with a bread knife, although, an electric carving knife would be better. I got mine at overstock.com
Overstock is way better than anything you will find at Wal-Mart, and in the end cheaper.
If your foam pads are shot, why not get one of the RV mattresses?
Memory Foam 5-pound Density 10-inch Full/ Double-size RV/ Truck Component Sleep System | Overstock.com
This way you could cut it out to shape and replace the foam in the cushions you have now. This gives you a couple of choices of foam thickness to match what was in there. You might even be able to use the remaining foam to replace the settee foam. That way when it was not made up you could not even tell, and you don't have to put away the topper.
One last thing, if you go to almost any goodwill they will have an electric knife for around $2. I think every married couple from the 50's to the 70's got one as a wedding gift and get donated quite often, normally never or infrequently used.