One other thing though.  While the quality and quantity of sausages on sale in the park are great, one of the traditions of the Brewers fans is to tailgate before the game.  Not sure how you were planning to get there but if driving is an option and you have a small grill you can bring along, it's a very festive atmosphere in the lots before the game.  They have plenty of places to dispose of the hot coals too.  If you can't do the grill, then just pick up some sandwiches and a cooler of beverages.  Should you decide to go this route, the website usually has information about when the lots open for tailgating.

the best part of the park is the tailgating--there aren't many baseball parks where its easier to do or more celebrated than there. and the cubs series really packs the parking lot with people. its lots of fun, and the brewers and cubs fans get along very well, in my experience. nothing like cubs/sox or cubs/cards.

opening day and the cubs games are generally the only ones that come close to selling out. there is a saturday night game and a sunday day game the first weekend in july. i would bet those would sell out the first day of sale, or close to it. if yo0u want to go one of those days i'd get in right away.
for the other cubs games, the outfield bleachers and primo tickets usually sell out fairly quickly. but you can still get other tickets weeks later. the upper deck behind home plate and on the baselines is a nice value and has good sitelines. i usually go after the "field outfield box" seats. they are between third base and the foul pole. decent price and great view/close to the field.