I have a salt water pool with a slide. No problems here.
I for one don't see how you can build that pool for 20-30k. In our area, the basic gunite pebbletec pool started higher than that.
As for quote 2, he should be able to estimate the amount of retaining walls he will need to build the pool. I would not leave that up in the air after the contract is signed. Our PB did say we would need fill dirt at $50 a truck and that was left in the air, but he ended up bringing in about 18 truckloads and never charged me for them.