Seller financing down payment - deal term ideas

[Kelly P.]

How do I structure the offer asking seller to finance the down payment?  

I qualified for a mortgage (excellent credit; solid W2 income), but don't want to use my own money for down on 2nd home. 

Seller has $175K mortgage (or less)

Offer full price $334,000

What is the structure of the 20% down payment financing?  (5 years, 5% non compounded, balloon, etc.)?

Do I have to ask permission from my bank? 

In case this matters: Seller also owns the other unit of the 2 unit townhouse and I'd like to buy that in the near future (I assume seller will want to sell once lease expires on 2nd unit)