Don't overcomplicate the issue.
Sure, you can buy her home without a downpayment. It's easy.
There's no law that says you have to apply somewhere for a mortgage. Just make it a private transaction between you and her.
Basically, she'd be the "bank." There'd be a mortgage, but every month you'd just pay her. She could sell it to you for whatever she wanted--$280,000 for a home valued at $330,000 is fine. Just running the numbers quickly, on a 30 year mortgage at 6%, you'd pay her $1,679 (principle and interest). You'd also be responsible for the property taxes. And that'd be with no money down; she'd create a $280,000 mortgage.
If you sold your current house, the question would be how much equity do you have, and how much of that would you plan on using for a downpayment? If your mom agrees, you don't need any downpayment. But let's say you put 10% down. So you'd be finanncing $252,000. In that case, at 6%, 30 years, you'd pay her $1,511 every month.
So, unless she wants all her cash up front, don't hassle with getting a mortgage from somewhere else. Save all the junk fees and unnecessary expenses. You'll probably have some costs--most jurisdictions usually have some transfer tax. But just go to a good real estate lawyer and he/she can draw up all the papers.
Also, your mom should check with an accountant regarding all the tax implications that might be involved.
Hope that helps.