The highest amount to deposit in Luno is N200,000.
The deposit limit depends on your verification. There are three verifications on Luno for deposit and withdrawal
For level 1: Phone verification: You can deposit/withdraw up to 200,000 naira
For level 2: ID verification: You can deposit/withdraw up to 500,000 naira monthly
For level 3: Proof of residence: You can deposit/withdraw up to 10 million naira monthly
Luno charges N 150. For trading fee and N 200. For withdrawing directly to your bank account.
For makers, trading fee is free on Luno.
For privacy sake, I will prefer p2p exchanges like LocalBitcoins, Remitano or Paxful. It is also said above, cryptocurrencies are legal in Nigeria, and you can purchase directly from exchanges that support credit cards like Binance.
P2P marketplace and Bitcon ATM is the best answers to buy and sell Bitcoin without submitting KYC
I do not know much about bitcoin marketcap in Nigeria but there are p2p exchnages like paxful, it is frequently used by Nigerians, but someone needs to be careful as some people use it for scam purpose, they must not release the bitcoin from escrow until they receive the money for bitcoin they are selling like you said above.
This may also interest you
Nigeria is among one of the countries that show huge growth interest in cryptocurrencies, according to coinmarketcap.
9 Countries Show Huge Growth in Cryptocurrency Interest: Coinmarketcap
https://news.bitcoin.com/countries-growth-cryptocurrency-coinmarketcap/