I wrote a post on spare wheel sizes (normal and space saver) some time back, have a look for this. I know for SURE that a 19" wheel will not fit in the spare area, I believe this may be the case for an 18". Easiest is to put your car on stands, remove a wheel and see if it fits (the issue is wheel width, so that you can put the trunk cover in place).

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2002 Euro X5 (E53) 3.0 diesel (M57)
5 speed manual with UUC short shifter.
Pre-xDrive NV125 transfer case (chain & gears).
Factory Xenon headlights, Spyder LED tail lights
Updated vortex crankcase breather.
Motorsport dead pedal, factory fire extinguisher, factory first aid kit, factory F&R mud flaps, factory PDC

Bilstein B6 shocks F&R
Redline oil has replaced "lifetime" fluids (F&R diff, manual transmission, transfer case and P/S).
Dimple magnetic drain plugs in all compartments.