I also come from a lightweight backpacking perspective (trailname Garlic, PCT'04, CDT'07, AT'08) and a few years ago came back to bike touring on my '90s touring bike. I upgraded wheels inexpensively by shopping used bike parts in shops, Craigslist, and friends' garages ($40 total--got lucky). I carried the same weight you're planning on a 4500 mile tour a few years ago with 36/32 spoke rear/front and 25mm wide tires on 21mm rims. I was unhappy with Schwalbe Marathons on that tour--way too heavy and stiff, and now use Conti Gatorskins. I really enjoyed the light wheels and skinny tires with a light load (but I ride almost all pavement).