Weather on the east side is much better than the west during the spring thru fall, but winter? not so much. but in the winter the I-5 corridor can get pretty nasty too. Mt. Shasta always have snow during the winter, as does the Siskiyou's passes. But the east side can get to 120 degrees in the summer, this can cause over heat problems in some vehicles.
In the spring when the orchards are in bloom is my favorite time to travel thru the Okanogan valley.
There are only two hills that would give anyone a hard time, Starting at Weed, you will be on gradual climb into eastern Or. then a pretty easy let down to the Columbia river crossing, then a hard pull (4%) out of the valley. then an easy run up to Blewett Pass, that is 3-4% rise for 5-6 miles, then a 5-6% down grade into Wenatchee. then it is an easy run to the boarder along the river.
side trips to Leavenworth and Grand Coolie are always fun.