If you want really floaty and really slow, check out some of the ff towline gliders at outerzone.co.uk Of course most of them are TOO floaty for an RC glider. Another place you can look is here:
I agree that Profili is very useful for evaluating and plotting airfoils.
You can learn a lot about design here:
You might think about a larger Apogee. The small ones stay up ok, but they're fast for their size so I wouldn't exactly call them floaters.
You can learn a lot by looking at one of these:
You can learn even more by building one. I owned one for a few years and they fly very nicely. Float quite well but also have reasonable speed range. And they're simple. And inexpensive.
Any reason for the 1.5 meter size? They float better when they get larger, usually.