As @Jay Hinrichs suggests, you cannot get a regular consumer mortgage for this project, which will add expense.
The real value here is that your mother is going to sell you the property for much less than its worth. Think of it this way: if you had $150k but no lot (let's say that's the market value of the land), would you go out and buy a lot and build a house?
I would encourage you to not have financial responsibility for this build and sale being a success. If you sell, I think you will likely end up with more money than if you to figure out real estate development from the (literally and metaphorically) ground up. You will also get more money if you offer it for sale with builder friendly terms such as carrying the mortgage yourself during development, but you will have to wait to get paid, and that brings us to the main rub: getting your mother paid. Does she want the $50k now? If so, you may be forced to take a private loan, or see if a bank in your area will give you flavor of construction loan. If there's a way to avoid having to transfer it into your name and then sell it again, that should save you money as well depending on your state's taxes.