Kendall,
Nothing wrong with a commission-based job. You just have to be able to provide documentation for the commission.
A lender will average your salary/commission for the past 2-years. You might have to provide a Letter Of Explanation (LOE) from your employer stating that your commission is normal and will continue for the next 12+months.
You can get conventional financing on a duplex with as little as 5%-down - But with less than 20%-down, expect to have mortgage insurance (PMI).
I have never used a job-offer letter in the 15-years I have been doing mortgage lending. That being said, your money (no pun intended) may vary. However, the job would have to be within the same line of work or industry as your current job.
Lenders will use leases as a source of income but generally only if there is a preceding rental history (and usually 24-months at that). Without prior rental history, you probably won't be able to use rental income to qualify.
I've got a great lender here in MO that I can refer you to, should you want to get pre-qualified (which you should).