I don't know about you, but the PVC valve I use was just a charm, although it can be bulky.
I attached one to my pump inlet for a drain point. I put a threaded joiner at the inlet (female NPT, 1/2), to which I screwed a threaded CPVC tee. I used another threaded joiner to attach the bottom of the tee to a threaded PVC ball valve.
The PVC ball valve can be pricey, but 4-5$ isn't too bad, is it?
As for the galvanic corrosion, yes, everything will interfere: brass on copper, copper on stainless, stainless on brass...
Look at this chart
Copper and brass are pretty close to each other, so the effect will be very small.
Stainless steel on the other hand, has a nickel plating, and that's some way down the chart. It's not as bad as aluminium+copper, but it's there. Stainless steel itself is found throughout the chart, because there are different compositions of it available.
If I was you, I'd stick with the heatercore. I can look one up for you, if you give me a size.