You need a lot of people to handle the process of buying land, building on it, and selling the property.

You need a lot of people to help you to handle the process of buying a piece of land and to build a house on it, which you will later sell . In order to handle the process of buying a piece of land on which you want to build a house that you will later sell ,you need a lot of people to help you.

I've given you a more natural, native way to sell it but your original sentence was good. You need to say "land" or "a piece of land" because this meaning of "land" is uncountable. The relative clause ("who....") was not a bad option but the better option, in my view, is a purpose clause beginning with an infinitive (to + verb).

Thank you very much :)