Op might get an information overload with all the suggestions and recommendations in this thread.
There are different ways to buy or sell Bitcoin;
• Exchanges: this is a platform that connects buyers to sellers and could be centralized or decentralized.
Centralized exchanges and the name implies, control the funds you deposit on their wallet and can freeze or lock them if you violate their ToS.
Decentralized exchanges on the other hand do not have access to your funds, but only connect buyers to sellers.
• Peer-to-Peer; this could indicate a face to face trade between two individuals or it could be done on a P2P platform, this platform could be centralized or decentralized.
• Bitcoin ATMs; this is another way to purchase bitcoins, but I would not recommend it due to the security risks involved with publicly buying Bitcoin.
My recommendation would be a decentralized P2P platform like Hodlhodl or Bisq, if you're privacy conscious.
Will I be able to purchase the bitcoin straight away and then use this to make the purchase without a delay?
tl;Dr
Yes, you can, you just need to select a means to do that.