Hi
Congrats on your purchase.
Services on the 118i are parts/component based, check your OBC to confirm what needs servicing.
I previously owned a 2005 118i and had it was serviced at the dealers.
Stuff like oil , filters , plugs , brake and coolant fluid are quite reasonable.
Brake pads, disks and other should be done by a private workshop as dealers are way too expensive, scout around for pricing first.