Your green card sponsoring employer will need to go through the recruitment process (can take 3 to 6 months), then apply for Prevailing Wage Determination (PWD) with DOL and PERM LCA before filing I-140.
The whole process up until I-140 approval may take 1 to 2 years depending on if there are any PERM audits/RFE etc. That is the reason why most H1B holder have their employers start green card process in their 2nd or 3rd year of H1B term.
Yes, the beneficiary need not be present in the US for the employer to start the PERM process. Once I-140 is approved , your employer can file cap-exempt H1B and you can travel to the US to work for the employer.