Many people do a sour mash but in my opinion you should go with a known culture as opposed to just relying on the lacto that is all over the grain. If I were you I would exclude the cranberries initially, make the wort keeping the IBU's very low (5-10ish) and do a 90 min boil to drive off DMS and also drive off as much hop flavor and aroma compounds. They do not compliment sour ales like this very well at all. Cool to 70F and pitch a Wyeast Lambic Blend and park that bad boy in the dark for about 6 months or until the pellicle falls keeping the ambient temp between 60-70F. Then place the cranberries into another carboy and rack the beer onto them. You will want to use about 5-10 lb in a 5gal batch. Let it sit for about another 3-6 months and you will have yourself a damn fine sour ale worth bragging about! Good luck in whatever you choose to do.
Proud supporter of Brett, Lacto, Pedio and all the other critters your brewing friends warn you about.