Knight 209 FPJ plunger guns come up for sale in descent shape for $150 or less. SS maybe a bit more.

As for BH209, ive done it and don't recommend it. 80gr seemed safe but 100gr would pop the FPJ off with some bullet/sabot combos. Its probably better to use more T7 or whatever powder and get a bit more fps safely. You dont have to use fresh ones but i would for hunting. Remove the primer and put them in a basket in the dishwasher.

I wouldn't reuse them much though.