Buying used climbing equipment is very dangerous. This stuff does protect your life, after all. You never know if there are cracks, flaws, or some crap in the moving components.
I have found the best way to build a trad rack is wait for a big sale (Christmas or other holidays) and then buy a large amount of new equipment at once. Be patient. You want at least 30% off if not 50%. Yes it's expensive and it sucks, but this equipment should last you at least 20 years. Resist the urge to "upgrade" your old cams and nuts when new stuff comes out later on.
BD cams are in fact the best, but you don't necessarily need the best. Metolius has very good cams (second best IMO), while Trango cams and HP link cams are also nice. Don't buy Wild Country. I hate their gear.
Nuts are a very nice supplement to cams, but don't think that you can just substitute lots of nuts for a good rack of cams. Nothing is as reassuring and secure as placing a nice big cam in the bomber place. You can't get buy with nuts alone.
Pro Tip: See if you can find "factory seconds" for sale. These are pieces of gear with cosmetic defects, such as the paint color not being quite right, but nothing wrong stucturally. They are often 40% off for that silly reason. I love buying "factory seconds".