Visa - exempt is when you arrive without a visa, but are admitted to stay due to your nationality...Americans receive 30 days. A re-entry permit allows you to leave Thailand and return and your Visa will still be kept alive, even though you temporarily left the country.
Thai Immigration requires Ca$h, but it can be in any exchangeable currency and not all in Thai Baht. The chances of you being asked are slim, but better to have it "just in case" because if they ask and you do NOT have that amount in cash, you can be refused entry and sent back to where you came from.