Personally i wouldn't even bother flashing a slim. Updates when new firmware comes out takes awhile, you need to replace the Motherboard of the dvd drive. Its just going to add up, and then if one day it gets banned, you spent alot to play offline? Personally, and this is my opinion, just do, what I do, I get cheap xbox's from places such as flea-market or eBay stuff like that, and just flash those. I spent $50 on my falcon with PSU and av cables, and i had for a year now. I think if you do get slim its good to RGH it, or keep legit.
Now if you actually want to flash it, here is the drives you should look out for. You should try to get a Liteon DG-16D4S(FW:9504,0272, 0225,0401, 1071)but most likely you'll get a GD-16D5S, which at the moment has no firmware made for it. And there's also a Hitachi 500/502 DVD drive, but those are less common, but haven't been flashed yet either. Now you could get a slim and use an xKey as it works with all current drives i believe. But I'm not sure its really live-safe. I think that's every drive then. If i missed any, tell me them. (The more info the better). If you get a phat xbox, Hitachi 0078/0079 have no LT3.0 yet. Everything else tho is fine. I hope this is helpful to you. I do not know how to check what drive your getting in slims, in the phat you can check by removing the face-plate, and looking at the hole under the drive, and see what color wires it is.