job offer letter should be addressed to you with the details of all benefits
, which you should get it when you get the job. it should not have certain
time limitation if you plan to apply for Eb-1B to prove it is a permanent
job.
employment verification letter for I-140 should emphasize that the job is a
permanent job, without any time limitation on this position.
EVL for I-485 is required to prove the job you used for I-140 application is
still available for you at the time you apply for I-485. The salary has to
be included in the letter.