Most Brokers can easily handle the written task of writing a lease-purchase (a.k.a rent-to-own). The problem being that most Brokers, myself included, typically avoid these types of transactions like the plague because they have a very low possibility of closing. More often than not the consumer defaults, gets relocated, gets "sticker shock" once they apply for loan, etc. So, when it's all said and done, the Broker your elected to hire most likely spent a fair amount of time finding a home, negotiating the terms, collecting keys, dealing with tenant objections, etc. to all of a sudden, after 12-36 months, not receive compensation for any of their efforts.
I realize that my response puts a somewhat sour outlook on your proposition but, based on the terms your propose above, in the areas you are looking to reside, my above mentioned outcome has a better than 50/50 chance of being the case (especially in the eyes of a Seller).
All that being said, I would ask the question "Why are you looking for a lease-purchase instead of simply buying?" If credit is your concern, my best advise would be to speak with any reputable lender in the Seattle area as they can most likely easily assist you with any repairs that may be necessary. Next question, if a down payment is the problem; it's going to be a problem with a lease-purchase as well (Sellers need/want/require some form of consideration to remove their home from the market and/or elect to take the risk of going through a lease-purchase).
In the same breath I would suggest looking at down payment assistance programs like a "House Key State Bond Loan" provided by the Washington State Housing Commission (http://www.wshfc.org/buyers/key.htm) or, alternatively, there are ARCH communities all around Seattle that may be of interest (http://www.archhousing.org/HTF/).
Last but not least, the best Lease-Purchase Broker in Seattle, that I am aware of, is Kassandra Rose and she can be found at http://www.onesourcerentals.com/
Ian, I hope some of this was helpful. Having not spoken to you about your topic I am, for lack of a better way of saying it, shooting from the hip. If you have any questions, please feel free to contact me.
Justin W. Richards
Asset Realty Group - Kirkland