One problem is the threads will likely be corroded to some extent and you can't get a good "feel" of the tension.
I'd recommend you get a metal bristle brush and clean the spoke threads and use new, lubricated nipples.
That will help considerably.

I've salvaged wheels for CL flippers, using used spokes, rims & hubs. It's kind of a pain and takes a lot more effort to get even tensions etc. but if you have more time than money, go for it. I gave up trying used nipples though.