I've never used German silver, so I'm no help there. I'm no expert, but I'd be happy to share what what's worked & hasn't worked for me...
Artistic Wire and copper wire are fairly inexpensive so they're great for practicing and experimenting. Copper has become more popular as the prices of silver and gold have risen, so I've used copper in finished pieces, too. The copper I've used will darken with time so I try to use a mix of bright copper and antique copper beads and spacers in those pieces so that everything blends. It seems like I read or heard about copper wire that's been treated to delay darkening - maybe someone else can help with that.
I love working with gold filled wire, but it's pretty expensive so I end up using a lot of sterling just because of the cost. Argentium silver doesn't tarnish as fast, but all silver will eventually tarnish if it's not cleaned or wiped down. Silver has a different feel than GF though - kind of hard to explain.
Aside from price, you want to look a 3 things when you buy wire - shape (round, half-round, square etc) gauge (diameter) and temper. Temper tells you how stiff or malleable the wire is - you'll usually see abbreviations in catalogs - ds= dead soft, hh= half hard, h= hard. They all have their uses, but generally the bigger the wire, the softer you want it to be. When possible, you don't want to buy wire on a spool as it's harder to straighten without work-hardening it. And tools with nylon jaws go a long way in preventing scratches. You can also put masking tape over your regular tools.
As far as suppliers go, it pays to shop around. Call for current prices before you buy because the prices of precious metals fluctuates. (A lot of catalog and website prices are based on prices that may or may not be anywhere close to reality. I saw a site the other day that priced their silver based on $6 per ounce - silver hasn't been that low in years.) Try Rio Grande or Monsterslayer for wire. If you buy wholesale, also check Foreign Source. You'll get good prices at shows sometimes, so check them out if you have any near you. There are tons of places to buy wire - maybe someone else will post their favorites as well.
Hope this helps!