1) Do you have an audience ready to use it? This should be the priority. Build the community before you spend heavily on a platform. Why would you launch a platform for a community that doesn't yet exist? That's a huge gamble. Use something simple to get started and invest more as your community grows.
2) Is there a better use for the budget? Most of the time, it's far better to spend the money on more/better community managers than a platform. It's not the platform that determines the community's success, it's the community manager. I've seen communities thrive on terrible platforms and fail on terrific platforms, but I haven't seen many succeed with terrible community managers. Nine times out of ten, your budget is better invested in community managers that do things on the platform.
3) Does it have the right, few, features? This is a typical situation, an organization lists every feature it wants the audience to be able to do then looks for a platform that has those features. You can see the problem: the audience won't use those features. You won't know what features the community needs until you have an active community. In most communities, only 2 - 3 features are used. You don't need a platform that offers as many features as possible, you want a platform that does specific features really well. The minutia of the discussion boards should be of great interest to you. The fewer features you use, the more concentrated your activity (LINK) is. That's a good thing.
4) Can you easily change the platform? You might think an organization wouldn't invest $500k on a platform without realizing it will need continue maintenance/changes, but then you would be surprised. This isn't a single cost, you need to be able to change and adapt the platform to suit the community. These costs can rise extravagantly.
5) Does it feature activity above everything else? Don't create a content site as a community site. If it's a content site, feature activity for people to read. If it's a community site, feature activities which people to participate in.
6) Do you have a HIGHLY trained staff for it? I threw this one in there at the last minute. Last week I spoke to an intern managing a community that uses an enterprise platform. Even she agreed it was madness for her to be responsible for such a huge investment.
Don't spend the majority of your budget on the community platform. Start simple, grow slowly, invest more as the community grows. Gradually you learn what the community needs and you can add those features.
(Image via The Noun Project)