..........for a 7 speed 26" mtb. The bike came with single wall rims. I want double wall rims and a freehub. Weight is not an issue. I weigh 185 lbs and this bike could see some use on singletrack, but mostly it will be for unpaved double track. I want a strong rear wheel. I'd like to keep the cost under ~ $150.
Any suggestions?

Will this set take a 7 speed cassette, and are they double-walled?
https://www.nashbar.com/bikes/Product...2_10000_200472

Probably is a double wall............haven't seen many not anymore. As far as a 7 spd. compatible???? Call them and I am sure they can steer you in the right direction.

If you are NOT running disc brakes, run a Wanted To Buy ad on CL. There are lots of good wheelsets out there for cheap that do not have disc compatible hubs. Many riders just replace the entire set when they upgraded to disc because the cost of relacing was more than buying an entire wheel set. I've picked up three sets of Mavic wheels including a set of 517's and have yet to pay more than $35 for the set. Used ALEX wheelsets are even cheaper.
You may have to be creative and buy some spacers to make the 7 speed casette work on the 8/9 speed freehub, but your LBS can help you out there.