In my opinion the passive imperative suffers from similarity to the agent noun derived from the verb.
This usually happens in the first conjugation.
That is, the imperative meditator ("plan!") looks like the noun meditator ("planner"), and vowel lengths match as well.
My first reaction to this word is that it's a noun.
Therefore my starting point to choose a phrasing that makes this reading less likely or less harmful.
My immediate reading of meditator ut consequatur is "so that a planner would achieve".
The imperative reading might not occur to me at all unless there is a clear person being ordered.
For a motto it sounds weird to say "may he plan so that he achieves".
Therefore I don't quite like the third person option, although it is grammatically perfectly valid.
I think a motto with an order works better when aimed to the reader.
The second person motto meditator ut consequaris can still be misread as "so that you would achieve as a planner", but that is less likely than with the third person.
And that alternative reading isn't all that bad; giving two legitimate but different readings with essentially the same message is a nice play with words.
I prefer the second person option.
Just because the future imperative is ambiguous, I would consider changing it to the usual imperative: meditare.
If I may suggest an alternative wording using these words, I think a more natural choice is to use a gerund to express the means.
This makes the roles of the two verbs abundantly clear, and it sounds somehow more Latin to me in style:
You will achieve by planning.
As to whether meditari and consequi are the best verbs for your purpose, I will have to point you to an online Latin dictionary.
Whichever verbs you end up picking, my recommended structure is this last one with a gerund and a future tense.
Of course, the ablative gerund can be replaced with the ablative form of a noun if the means can be described well with a noun.
Bear in mind, though, that a gerund will make it more active.
To me meditando gives the impression that you actually have to plan yourself, whereas consilio just gives the circumstances and makes me think you are working on someone else's recommendation.