Private Mortgage Insurance... you typically have to pay that if you finance more that 80% of the purchase price
This is absolutely true ... but there are ways around paying PMI if you can't put down 20%. They'll do what they call 80/20 loans ... or if you can put down 5%, an 80/15/5 loan, or if you can put down 10%, and 80/10/10 loan .... you get the point. But what they are doing is financing 80% of the price on a primary mortgage at a competitive rate. The 2nd mortgage is the remaining amount ... 20% if you put nothing down. The rate on this loan is a higher rate because it is considered an unsecured loan, and a higher risk for the lender. That's the bad news, and you can think of the higher rate as effectively paying PMI. The good news is that there is no PMI to get rid of ... you simply pay that loan off.
There's lots of good info that people are throwing out there for you ... but you need to run the numbers for yourself and see what makes the most sense to do. A house is a poor investment relative to your 401k, stocks, etc. That being said, you still need a place to live, and if you want your own place, that might be worth more to you than a return on investing money that could potentially be applied toward a house.
There's a school of thought that by borrowing against your 401k, that you're being taxed on your income twice ... once to pay it back, and a second time to withdrawl when you retire. I can see that, but I borrowed against my 401k anyway. They allow this because if you default, then they turn your loan into a withdrawl, but I'll bet you'd be forced to pay some 10% penalty.
Use the PMT function in excel to estimate loans ... it's quick and easy to get numbers for comparison.