The XP 400/4000 would be your best bet. They have a more powerful motor then the 200/300, so it won't bog down on cutting heavy metal and such (at least in my experience).
Definitely invest in the 4000. My father has 3 dremel tools from the 1980's or earlier, and they are still running strong right up till the day I got a 400, and bought my father a 400 as well.
You should also compare prices on E-bay for dremel "kits". I bought the 90 piece kit that contains a ton of attachments, a sawz-all, planer, flexi-shaft, dry wall cutter, roouter, and a few other things for ~$120-$130. Not a single item was used, and every box in the tool kit I received was in brand new plastic wrapped packaging.
Here is my must have list for the dremel:Flexi-shaft:
Extremely useful for tight workEZ Lock
- Heavy duty cutting wheel. Very well built, and while a little pricy, I have found them to last longer then the equivalent number of #420 Heavy cutoff wheels. In addition they are more forgiving then the #420's, because if you slip and twist the disk at all they won't really explode everywhere like a #420 disk will.9902 Tungsten Carbide Cutter
: This tool has been used on at least half a dozen projects to deburr a metal surface I cut. It last longer and seems to work better then the sanding disks for deburing metal and taking sharp edges off metal.
I'll add more items as I think of them.