I've been inspired by good people (nairalander). I'm ready to build my house. I want to build 5 bedroom duplex. I don't know anything about building. So I need help.

I got land somewhere at dalemo area of sango ota (about 7 street from gateway hotel). It is 50 by 100 according to the owner of the land that want to sell it. He want to sell for N3m.

My questions:

How do I know if the land really worth that. Who should I invite as experts to complete the land buying deal.
What else should I do before buying the land.
And what should I do after buying the land till I have my house.

I will be glad if I get step by step guide possibly with recommendation of experts to contact.

Thank you Nairalanders