Hello, I have a 401K worth roughly 30K, but I am interested in cashing it in to assist in the down payment of purchasing a house. What kind of penalties/taxes might I encounter and is this a really dumb idea? The other part of the money would come from selling stock, which I know there will be a capital gain tax on.

Thank you

If you withdraw money from 401K, you will pay 10% early withdrawal penalty and the total amount will be tax as your ordinary income.
Another options is to borrow money against your 401k. Check with your 401K administrator.
Still another option is to roll over the amount to Trad IRA. Withdrawal from Trad IRA for first time purchase of house is exempt from early withdrawal penalty. Read about IRA Your U.S. Tax Return: Traditional IRA and Roth IRA

MukatA's advice is sound, with a minor omission:

There is a $10,000 limit on the amount that can be withdrawn from the traditional IRA for the first-time purchase of a home.

The reason this is a dumb idea is the amount of money you will be losing more than offsets the value derived from it.