UK-based freelance invoicing overseas

Shall I add VAT,

No, why would you? VAT is a EU concept and a consumer tax for EU residents (ignoring details because they are irrelevant for the question).

Last time I checked the Dominican Republic is not in the EU - so VAT does not apply. YOu essentially are exporting out of VAT territory. This is not subject to EU VAT regardless of the type of client. VAT only applies to EU - not even EU Economic Area.

any other important thing I have to consider?

Freelancer: ASK YOU ACCOUNTANT. Not have one? Well, there is bad and there is legally not smart. That is that. Why? Because with an accountant ANY error is HIS fault. He has insurance AND - you have plausible deniability (as in: he makes a mistake, the government may not fine you to start with as you can prove you had no intent).