I am a CPA, but you should contact your personal CPA to make sure that you explain all the details to make sure they assess your situation correctly. That said, a few individuals above mentioned that you get a step up in basis at the date of death. This is true. What does this mean. The day your mom would die, you would want to get a FMV of the property. The FMV then becomes your basis in the property. Therefore you would depreciation the new basis over a 39 year life (falls under commercial at this point) excluding the value of land.
Each situation is different, but passing property or other assets at the date of death are great ways to pass on valued items while getting a step up in basis. However in your mothers case we step into the area of estate planning. As of current laws today (i know there were proposals but they are not law yet), if her net worth is not more than 5M then there could be some different planning to be done. If that value is not close to 5 M then setting up an agreement with all parties so that you get the property at the date of death might be best (please consult with an attorney on the agreement).
If you get the property as a gift, then you inherit her basis or the FMV at the date of gift. It is a lesser of the basis or the FMV. In real estate, a lot of times the basis will be the lower if it has been held for a while. So if she has owned it for a long time, then her basis might be minimal or close to zero. If you would ever go to sell it you would have a large capital gain. Now there are options to post pone this tax, but it would depend on the tax laws when you sell the property.
If you buy it from your mother, then we get into a little more complicated issue. Not saying it won't work, but you would fall into a related party issue. It would potentially only affect your mother. If the sale produced a gain then she would report and pay on the gain. If it reports a loss she would not be able to claim the loss. However the sale needs to be at an arms length deal. It would be advised to get an appraisal on it (potentially two to truly show its an arms length deal but thats for you and your CPA to decide) to show you are buying it at FMV.
Once again, please consult with a CPA that specializes in tax planning, because each scenario is different.