I can vouch for the Dell Ivy Bridge and Sandy Bridge Lattitudes. E6220,E6320,E6420,E6330,E6430.
Bluetooth is the only thing I have not had working.
Avoid any of the NVidia embedded models. Many times you want to look at the Dell service tag and run it through Dell support and check inventory of the machine. The NVidia versions run hot and battery life is bad. On the regular models about 7-8 hours with some tuning.
The illuminated keyboard is the shiznik.
Even on models without backlit keyboard you can add them. All the connectors are there and the bios picks them up.
I have swapped processors, ram, add cellular modems and SSD. They ship with Intel cards which are 3X mimo.
The expansion bay is quite handy as I use hard drive bay or dvd drive depending on need.
My only bad words are sound. The sound is not the loudest and crackles when jacked up. They used some puny sized amp onboard.
There are 3 mPCIe slots with 2 half sized, one with wifi, and one full sized mPCIe slot for the cellular modem. All models ship with a SIM slot. Parts on ebay are abundant.