Part of the issue is that you're trying to condense two questions into a single vote: "Spend XP Y/N" and "Spend XP How". That's easy enough to mitigate:
[] [Purchase-YN] Buy things
[] [Purchase-YN] Don't buy anything
[] [Purchase Plan] Buy option A
[] [Purchase Plan] Buy option B
[] [Purchase Plan] Buy option C
This allows users to write votes like "I don't want to purchase anything, but if we end up purchasing something, here's what I'd like purchased".
The next problem is that, because you're voting by line item, if two voting blocs disagree over what plan to follow, you'll end up purchasing the first item from
both plans, then the second item from
both plans, and so on and so forth. It is extremely easy to see a situation in which this completely fucks up the character's build. To deal with this, I recommend requiring voters to vote by plans, simply not considering any purchase plan that isn't formatted like this:
[] [Purchase Plan] Plan Red Ones Go Faster
-[] Buy option A
-[] Buy option B
[] [Purchase Plan] Plan not enuff dakka
-[] Buy option C
-[] Buy option D
Then you go down the winning plan and purchase things off of it.
edit: note that plan voting requires some special behavior on the part of voters.
Only the first definition of a plan counts; plans cannot be redefined, even by their original poster. Further vote blocks are counted as votes for that original definition
regardless of their contents. So these three votes:
Person A said:
[] Plan Foo
-[] Do A
-[] Do B
Person A said:
Wait I changed my mind
[] Plan Foo
-[] Do C
-[] Do B
Person B said:
I think we should do this instead
[] Plan Foo
-[] Do C
-[] Do D
Will end up being counted as three votes for the
original "Plan Foo", which has "Do A" and "Do B". Changing a plan requires going back and editing it in the first post in which it appears.