Others answered the other questions. But as to penalties, you will avoid them even if you underpay through withholding and estimated tax if:
- You underpay by less than $1,000.
- You underpay by 10% or less.
- You paid at least what your total tax was last year. If you made a lot of money last year (over $150,000), you’ll need to have paid at least 110%.
So you could owe tens of thousands of dollars at tax time but not get penalized so long as you make sure you covered last year’s tax bill.