I'll try and answer in the order of the questions:
The weight of the fly line should be matched to the weight of the rod. The line will cast best when paired properly with both the rod and your (developing) casting style. While a heavier line may be easier to cast to begin with (this is called "up-lining" the rod) since it will help to load the rod, it may also lead to problems down the rod (that is not to say that there are not reasons to up-line a rod at one time or another). Your best bet is to buy a line that fits best with the rod in question and learn to use the pair. You don't HAVE to buy expensive line or an expensive fly rod. That pflueger medalist kit will throw a cheap line just fine, and if you can get the fly where you want it with some practice and you can catch fish, it's good! Will you see a difference with more expensive line? Yes. Will you catch more fish as a result of that difference? Maybe. Is it necessary? Nope. If you can get a friend or a local fly shop to help you learn how to cast, do it. It will flatten out the learning curve a bit. And there's nothing wrong with practicing in the grass as long as you keep a leader on the end of the line (try tying in a piece of brightly colored yarn for some added air resistance and visibility to represent those poppers). Starting out learning how to fly cast without a leader will do some damage to the end of your fly line for sure.
For bass and bream, straight leader is fine. Take 6 or 7ft, tie a perfection loop in one end and loop-to-loop knot it to your line. Most my saltwater fishing I use 20# straight leader and it works just fine. You might want to use heavier than 8# for throwing poppers and such though, the 8# might not be heavy enough to transfer the energy from your cast down the line through the leader. In casting terminology, your leader would not "turn over" properly and your cast may fail due to the air-resistant nature of poppers. Ideally, you want the butt section of the leader (where it attaches to the fly line) to be similar in diameter to the end of the fly line. 8# is a little thin.
There is plenty you should know (and plenty of time to learn it), but very little that judicious use of the search function on this forum won't answer! And if something comes up, feel free to ask, there is a LOT of wisdom and experience on here to be tapped (I do it regularly!).
And finally, nothing wrong with being a bargain shopper! Don't feel bad about trying to save money, there's a lot of broke grad students and other fans of discount shopping on the forum too
Have fun learning it, and share with us how it's going! Welcome!