"Those who would be great must serve" I'd appreciate knowing how to say this. Its based on Matthew 23:11 (but not quite the same as the Vulgate translation).
If one is not restricted to the language of the Vulgate (it is unclear from the question if there is a requirement to conform the Vulgate in wording), for "would/want to be great" part, it is possible to use magna spectare which was used by Cicero(*); it means "to strive/aim for greatness" (literally: aim for great [things]), hence qui magna spectant
The "must serve" part can also be interpreted somewhat differently. It seems the more natural for this case is (as suggested in other answers) debeo; I think the subjective can also work here, like the famous Vulgatian fiat lux (Let there be light) - it is almost imperative in tone. But it seems the selection of this part really depends on what must means here, I would not rule out the original erit minister nor even esto minister (which is kind of future imperative). If we use debeo we end up with:
Ii, qui magna spectant, debent ministrare
(*) ... ii, simul ac iuvenes esse coeperunt, magna spectare et ad ea rectis studiis debent contendere (Cic.Off.2.45.4)
What does “those who would be great” mean, exactly? It's slightly archaic English and depends on context, but I think FlatAssembler has got it right in his answer: it means qui volunt magni esse. (You could also say cupiunt etc., but let's keep it simple.)
Therefore the most straightforward translation seems to me to be: Serviendum est iis, qui volunt magni esse.
However, as brianpck pointed out in a comment, this is ambiguous: It is easily read impersonally as: “one most serve those who want to be great.” It is therefore better to say:
Debent servire, qui volunt magni esse.
This is also somewhat more apt, as (or so my dictionary tells me) debere is used for moral duties, whereas the gerund construction is used for necsessities that flow ineluctably from the circumstances, which is presumably not the case here. (Although, to be sure, I take this differentiation to be more of a general guideline rather than an ironclad law.)
Incidentally, since “must” could also be past tense, a couple of other interpretations of the English original would be possible, but none sound very plausible.