I’m not a great moddler by any means, but I HAVE painted a lot of models since taking up the hobby.
For primers, I’ve never used anything but the cheap spray cans and I know quite a few moddlers who do the same even if they’re pro painters. You can buy about ten cans of the cheap stuff for every bottle of the Games Workshop stuff.
I have used Testors paints and did NOT like them at all.
I have used Games Workshop paints and I liked those. They’ve got a good range of colors and the colors are already shades that are good for this kind of painting. They’ve pretty expensive if you’re looking to buy more than just a few colors though. If you’re looking to get into painting seriously, enjoy painting fine details, and have the extra money to throw around, the good paints are something to consider.
I currently use the cheap hobby paints (Apple Barrel and FolkArt) that I’ve picked up at Wal*Mart. They’re almost as good as the good paints and like the primer, and it’s MUCH cheaper.
For the paints, since it sounds like you’re new to the hobby, I’d recommend picking up some of the cheap stuff, see if you enjoy the painting and see if you like the quality before you spend a bunch on better paints. The Apple Barrel stuff costs about 1/8 of the cost and you get at least twice as much paint.
Another thing of importance though is the brushes. I personally think the brushes are a bit more important than the paints. But you can still get off cheap. Look for a package of brushes, an assortment is good. I like softer bristles on mine though some people like firmer brushes. What’s important though is finding something with some nice fine tips. Take a look at my model thread and look at the first post. You will see some brushes to the left side of the image, golden bristles, blue handles on some, orange on the ones in the package still) Something along those lines. The small tip ones work great for detail, the wider brushes work for applying paint to larger areas or drybrushing.
Hope that helps!