When purchasing new maps, you need the "High" version, not the "Professional" version. Here in the US they can be purchased directly from the map supplier (formerly Navteq, now TeleAtlas) or from the dealer. Which has the best deal depends on discounts offered (here, many dealers offer a discount for BMW club members).

We also don't have TMC here, but I believe you need a disc with some sort of TMC enabling key on it.

You can run into problems with a very mismatched map and OS, because some combinations were not tested (new OS, ancient map) or impossible to test (new map, old OS). I have seen odd behavior with the new OS / old map combination - routes were very sub-optimal.

By the way, here in the US we didn't get DVD navigation until the 2003 model year (September 2002 production dates onward.