Okay, first of all why are you using RG15 for the airfoil? That is a very fast section. How much flying experience do you have? How fast do you want this plane to fly, and how gentle or forgiving do you want it to be? If you're new to flying, you definitely don't want to use RG15. If this is your first glider use an E205 or S3021, if it's your first plane of any sort then use a Clark Y.

What sort of power system will you use - what motor and how many cells?

A 110" wing with the RG15 airfoil is going to be very thin, you'll need more than just foam and balsa or it will fold up pretty easily.

There are plenty of published designs available, you don't need a kit if you can cut your own wood. But if someone else has already done the design work for a similar plane then it makes sense to build that instead, unless you really have a good reason for wanting your own design.