That combo would work well, but keep in mind that the slot-2 Supercards are not the greatest for GBA games. See http://wiki.pocketheaven.com/SupercardGBASlowdowns for more info on that.
Most people get a slot-1 device and a slot-2 expansion pack, but since you want to use this on a standard GBA as well this would not be recommended, as the expansion packs usually don't work well (if at all) without a slot-1 device as well.
Some of your other questions I can't answer due to forum rules, but the only slot-2 device to support SDHC is the M3 Perfect. However, this is not worth using since it only works with media files using the M3's built-in media player, not GBA or DS binaries. It's easier to act as if your max for slot-2 devices is 2 GB per media card.
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