I think there are a couple of important points to consider here when choosing shoes. You should have somebody analyze your stride to determine what kind of runner you are - by doing this you will be able to focus on shoe types that were made for you when you go into a store to make a purchase. If this is not possible due to where you live, etc there is a method you can do by making a 'foot print' (I can't remember exactly how they recommend doing this) and the features that show up will at least give you an indication of your posture, how your feet are planted on the ground, etc which would lead you to make valid assumptions on what kind of stride you might have. Also, look for wear on the sole of your current shoe as indicators of your pronation- greater wear on the treads to one side or the other might be a clue. Lastly, if you can get away with this w/out being sucked into some greater plan, visit a chiropractor for advice. I am sure they can determine this for you if necessary but it might cost you some money up-front (this is about as professional of an opinion you'll get, however)
Secondly, evaluate how serious you are about making running a commitment in your daily, year-round lifestyle. Shoes are not cheap and you may need to burn through 5 - 6 different pairs before you find the right one. i've been running consistently for about 15 years now and I found the Asics Kayano's were my shoe. I have over a dozen 1/2, 30K and marathon distances, as well as a number of international distance tri's under my belt and all have been run in Kayano's. I had to put down some hard cash to figure this out, but my knees are fine today and I have no idea what lower leg or foot injuries are like because i have the right shoes on. Granted, these shoes retail for about $130, and I cringe everytime I think about having to buy a new pair..... but they have never failed me. I have used every 'version' they've come out with since Kayano VII's (I just bought a pair of Kayano XIV - or 14's if you will- and these are my shoes for the Twin Cities marathon this year). These may not be the shoes for you, but my point is find the right shoe and stick with it. Also, when you find the right shoe check around at the end/beginning of the year to see if these are on a clearance rack. I've been able to get a few pairs of my shoe as low as $79 dollars so the store can make room for next year's model. Just a thought to consider.
My final advice is to make sure you are incorporating some variety into your routine to make your legs, calves, hip flexors, back, etc strong. Many people assume if they just run and run and run they will have strong quads, defined healthy calves and the excess fat on threir rears and tummy will magically disappear. This will happen to some degree but it will not cover core strength training which will target your overall ability to handle the balancing, posture and overall taxation you place on your body during the longer run and higher volume run periods. Besides, you are commited to put in the time to exercise - running is just one facet of many that will get you to the fitness goals you want to achieve. VERY IMPORTANT: do not use your running shoes for anything other than running. Yes, you can wear them around the house and such when they are brand new to break them in, but do not cross-train in them. You will greatly reduce the life of your investment.