You definitely want to try to keep weight down low when you are using a rack. If your bike has rack eyelets you would be much better off with a conventional rack. As far as handling, if you have everything hooked on nice and tight it will be fine. I did one grocery run where I put about 15-20 pounds back there. I got up to about 30 mph on that run and didn't notice the weight until I picked the bike up to put it on the porch.
You can get a decent rack for around 20 dollars. Just make sure you get the style that has the supports shaped like a triangle, not the ones with only one bar going down. For carrying stuff, some form of panniers are nice. I have the cheap Nashbar ones right now. They work just fine. I am planning on getting a set of Ortlieb rollers in a year or so. Like everything else in life, you get what you pay for.
Good luck and God bless!
Wayne J.