Structuring Commercial Lease Option??

[Thomas Fortune]

In need of a little guidance here: 

The company I work for currently rents our office space, and we are interesting in purchasing the building we currently occupy. I'm exploring options, and think that a lease with option to purchase in the next 3-5 years may be the best bet, but I'd love some other opinions here. A little background: 

Goals of purchasing: 

1) Build equity. Any cash flow is icing on the cake, but not the reason for the purchase. 

2) Purchase with the lowest down payment possible. 

Building details: 

Built in 2007, two story class A building. 2 units total - downstairs is 4,800 sqft and currently rented at $5,768/month. Upstairs is 7,500 sqft, and currently occupied by the owners company. We believe market rent for the upstairs is ballpark $10,000/month. 

Based on these rent figures and expenses, the current NOI is roughly $125,000 annually.

At a cap rate of 7.75%, purchase price would fall close to $1.6mm.

Seller would like to sell, but is not ultra motivated and won't accept lowball here. 

Structuring the deal: 

We can likely use an SBA loan here and do a 10% down payment (160k). Additionally, the seller would likely carry a 2nd...but i'm concerned we'd run into issues with debt coverage at that point.  (would like feedback on this if i'm wrong here!)

That's what brought me to the lease option - If we can structure a lease in which a good portion (or all!) of our rent check each month goes towards down payment, that would be ideal. I've just never done this before, and i'm not sure where to start. 

Thanks for your help!