Own NFT marketplace

Hello Harmony Supporters, Teams and all which are here.

I want to ask about Harmony network cost fees for own NFT marketplace.
I am planning to hire developer to build my own NFT marketplace. My main idea is to sell only NFTs. But huge size 1 million or 5 million at once. How much have to pay for fees and storage ? What is best option for IPFS platform ?
About smart contract what have to know ?

Thank guys in advance

Hey guys.

Someone can help me about that ?