I got a cheap "WaterMaker Mini" faucet mount RO filter. It's a bit slow -- 10 GPD -- but it was a good price. The water tests at about 35-39 TDS, but this is the initial batch, it's already been used (so should have already been primed) but it says to leave it on for 4 hours before using the water... So... maybe it'll get better?
But, assuming it doesn't -- is 35-40 TDS good enough?