Whether or not the property can be subdivided is something you need to know first. And then you will need to know the restrictions regarding the type of structure you can build on it. Go to the city/county building department and ask what the property is zoned for, what you can build on it, minimum criteria, etc. The city/county may charge for application for some of these services (permits, getting copies of documents, etc).
A real estate attorney can advise you and your mother regarding parceling the land and having you own part of it and type of ownership. And he can draw up the documents to make you a legal owner and have this recorded. He will most likely charge for you.
A lender can advise you on what type of loan you can qualify for to build. I don't think lenders will charge to get you prequalified and preapproved.
You would have to pay the attorney, which actually should be cheaper than an agent.