I just finished my first scratch built which I designed like you described. As you said, keeping the weight as low as possible is the most important factor. Mine weighed 4lb 5oz with a 53" span. I used the Laser 3D plans as my starting point but made many changes.

I don't think having wingtips makes a whole lot of difference. The problem with the Laser design was the ailerons didn't go all the way out; they stopped at the wing tip. That's why people were cutting the tips off. I put tips on, but also included full span ailerons. It looks good and they are also very effective.

I would encourage you to get a pile of 1/16" balsa, cut out a rib template, and start building!

Tom