If it helps folk, most pre-facelift cars are pre-wired for electric and heated seats. There's a cut-off at some point in the manufacturing dates (probably the same as for pre-wiring for cruise control) of around Oct 2005 when BMW stopped pre-wiring the cars for them.
I started with non-heated, leather seats on electric-memory bases, and bought heated, leather seats on manual bases to replace them - thus adding electric function. Swapping bases is easy - time-consuming and fiddly, but easy.
If pre-wired, it's as simple as plugging in the wires from the seats into the car's wiring and the heating and electric adjustment should all work. You'll also need a new switch panel that houses the roof controls, DSC/DTC switch and tyre pressure reset button. I picked up mine for £45 secondhand imported from the US, but obviously if you already have heated seats then you'll already have the buttons.
Not sure about cars without the pre-wiring though.
If your seats are post facelife fitted to a pre facelift then the airbag light is probably due to compatibility differences
I tried lots of things to clear the air bag light - note there is a big thread on this.
My solution, A small piece of black insulating tape over the airbag light on the dash.
I understand the airbags will ALL fire in a crash.