Hi @Genry,
Welcome to the community 
I recommend going through: Points to consider when creating a fungible token (ERC20, ERC777)
Once the crowdsale has ended you can call finalize, this will transfer the funds or allow refunds if the goal is not met.
You should create unit tests to check this.
You should use RefundablePostDeliveryCrowdsale rather than RefundableCrowdsale
Note that if you allow tokens to be traded before the goal is met, then an attack is possible in which the attacker purchases tokens from the crowdsale and when they sees that the goal is unlikely to be met, they sell their tokens (possibly at a discount). The attacker will be refunded when the crowdsale is finalized, and the users that purchased from them will be left with worthless tokens.
_From: https://docs.openzeppelin.com/contracts/2.x/api/crowdsale#RefundableCrowdsale_
RefundablePostDeliveryCrowdsale will refund participants if the goal is not met.
You should create unit tests to check this.
You can use IndividuallyCappedCrowdsale. For a minimum, you could check using _preValidatePurchase-address-uint256.
You should create unit tests to check this.