First deal- assigning Duplex, Help!

[Uber]

I am about to make an offer on a vacant 2/2 two story duplex (each story 1/1) worth around ARV 210k.

The owner estimates 40k on rehab. He is asking 135k. Owner financing is available.

The owner purchased it for 22k in 8/06 and put about 6k on rehab!

I will be assigning it to an unknown investor.

What should I do, make an offer? How much? How and what should I say that would help him accept a lower offer?
How will I fill out the contract if I don't know if the seller will be receiving all cash or owner financed? How much assignment fee should I ask for?

Thanks so much!