Yes, you'll need a slot-1 for DS stuff as the 3in1 can only run GBA homebrew and ROMs and it WILL need some sort of loader application to get stuff onto it.
If you want an all-in-one solution, then you'll have to go with one of the slot-2 M3s, EZ-Flash IVs, or maybe a Supercard slot-2 but the GBA ability of the Supercards isn't all that great as many games will exhibit slowdowns on the Supercard.
You'll have to make sure that the slot 2 is GBA size, or compatible with the original DS. It'll be fairly safe to assume any slot-2 cart with lite in it's name is only compatible with the DS Lite (because of size). As to DS compatibility no slot-2 cart was ever 100% compatible with DS ROMs, and likely won't ever be, and any DS ROMs WILL need to be run through the manufacturers provided software before it will run from the slot-2 cart.
Since the DS is flashme'd I don't believe that it would need a nopass or passme slot-1 cart to work, but you'll have to check on that yourself as I'm not positive.
Download play won't be too hot on slot-2 carts either, although since the DS is flashmed it MAY help a little.
Really, the best solution IS some sort of 100% drag and drop compatible slot-1 cart plus some sort of slot-2 cart for GBA specific stuff. This is basically what I have. You can see what carts I have in my signature, both of the slot-1s are good although the SCDS1 cannot run links/retawq (WWW clients) under dslinux. I have the GBA sized 3in1 and use Rudolph's app. They work great for me.
Vendors: well if you absolutely can't wait, you could check for other vendors in the stickied post at the top of the forum(below the sub-forum listings), but above these generic posts. Prices HAVE been going UP recently, e.g. I paid c. $22(Nov 2007) for my 3in1 from RHS, now I see that the same cart is $24. I also noticed that some of those super cheap slot-1 carts that I mentioned have gone up a little in price recently as well... (since February 08)
Really, it's hard to miss, but here's a link for you anyways
http://gbatemp.net/index.php?showtopic=33039