1) Wait until you're in Munich to buy the Lederhosen/Dirndl. The fit can be quite different from company to company and if you find it online for cheaper it will probably be poorer quality.
There are different quality and price ranges for tracht. Most expensive, and the best shopping experience, is Lodenfrey in the old city. Lederhosen start at 350€ and Dirndl for 400€. (Although last year they had some really nice Dirndl around 150€ for Oktoberfest.) More reasonably priced and by no means poor quality is Angermaier near Viktualianmarkt. Lederhosen start at 160€ and Dirndl at 130€. There are also some shops that sell a more costumey (and cheaper in price and quality) Lederhosen and Dirndl, such a Steindl Trachten across from Zum Augustiener. I think you can probably get a Dirndl there for 30€, but it will probably only last one evening. You'll find more shops pop up around the city as Oktoberfest approaches. There has to be at least 30 places in old city to buy Lederhosen and Dirndl, and that number doubles around Oktoberfest as department stores open up separate Tracht departments.
Also keep in mind, you will need to buy a blouse for the Dirndl separately (about 20-30€) and a shirt of the Lederhosen (35€-100€). The choice to buy is obviously yours, but you will find that in some tents, and especially in the evening, most people are wearing the traditional Tracht. I would say it definitely makes the experience more fun.
2) I'm sure you've read the weekends can be rough. If you arrive Saturday afternoon or evening it will be impossible to enter the tents. Sunday after 6pm you might have more luck. Maybe try going Saturday morning and if you don't get in you can wander around and then head back into the city. Then try again on Sunday in the evening. Definitely try to get in a tent and at a table, don't just hang out at the tables around the outside of some of the tents. Remember, you have to be sitting at a table in the tent to order a beer. Try to stay until at least 9pm in the evening, things get rowdy, but it is a very fun experience.