I'll list each of your wishes with the best choices:
NDS-GBA Linkage:
1. M3 Real with GBA Expansion, M3 Perfect, G6 Lite or Ewin Expansion Pack
2. CycloDS Evolution with EZ Flash 3in1 Expansion Pack
3. M3 Lite (and a NoPass if you need it)
DS Download Play:
There are many, such as R4, M3 Real and CycloDS Evolution
Nintendo WiFi Connection:
This is perfect for each and every flashcart.