I would start by simply asking that the seller assume a certain amount of the morgage without including any details. Let him come back to you. This will give you more negotiating room.
You are testing the water without giving any numbers. Since you have no idea as to what his response might be you do not want to be the first to state numbers.
My last purchase I asked for a take back of $150,000 and was hopefull I could get it for 7.5%. We finally settled on $285,000 at 7.25% with a long term and very flexable conditions. Turns out he wanted a no hastle steady income. Thankfully I didn`t give details in my first offer.
With todays rates you should be able to do even better.