You would need to get an attorney for sure, and one who is familiar with the finer points of subject-to purchases. As far as a broker, there's no need. You would approach the owner and establish this deal just between the two of you.
You asked about downpayments, and that would depend on what situation they're in. In one of the podcasts, a buyer set up a subject-to and it ended up being structured so the property owner wrote the BUYER a check for ~9,000 and then the buyer had the property and continued paying down the property. So she got a property without consulting the bank, got a property, AND got paid to do it.
That was BP podcast number two, I believe, with Karen Rittenhouse. She does subject-to and will walk you through the in's and out's of the whole process. Also, there are a few other podcasts with people who do a much better job explaining the process than I do. Have you checked them out? They're fantastic!