IMHO it would make sense to use the [[International System of Units]] for internal storage. It is not consequently used in other realms, not even in the German spoken countries (PS vs. kW for cars). Maybe it would be possible to use small scripts (such as WP-templates) to transcalc values, which can easily be developed by the community.
Internally we would translate it, yes, otherwise the numbers would not be comparable. But for editing we need to keep the unit of the source / of the editor, or else we will loose precision.
I propose to use a system which allow to store an SI prefix separatedly from the number and unit. Else we would have to express the proton (proton is about 1.6–1.7 fm) as 0.0000000000000016-0.0000000000000017 m and a fungus (2.8 km) as 2800 m. Especially the small values would be very error prone.
Allowing fixed order-of-magnitude prefixes would ease both error control and presentation.
(ASIDE: Regarding presentation: it is not always algorthmically eay whether to present 0.00000000000001 m as 1 * 10e-14 or a 10 fm = 10 * 10-15. In a scientific context, only the SI steps should be used, in another context the closest decimal may be appropriate.)
Gregor