SEZ SALE export

yes export lut is important, otherwise gst igst applicable, u ask sez dealer whether LUT FY 19-20 is available. no need not collect any documents

Yes... 1st of You will collect SEZ related certificate from the dealer who is in SEZ zone...

&

If You will supply to SEZ then consider if it's under LUT or IGST. If You have LUT then no need to charge IGST in Your invoice...
If You don't have LUT then charge IGST in Invoice. And the IGST will be payable by Your pocket...

No ... I don't think any documents required in case to supply to SEZ , Yes In case Refund is claim against supply made to SEZ then following needs to collect from SEZ

Proof of receipt of goods or services as demonstrated by the specified officer of the SEZ.
A statement containing the number and date of invoices, the evidence regarding the endorsement SEZ Officer and the details of payment.

Refer Rule 89 of CGST Act

Thanks a lot