A couple of points:
You say you are a small community "with no amenities or common ground apart from the car park". (No lift? A cleaner for common parts?) I wonder what exactly, therefore, these presidents are spending the money on? And more to the point, what money? Do you not set an annual budget at the AGM? Are your contributions not based on this projected budget? Are they spending more than you budgeted for, without prior agreement?
As for the decisions on how to use the community funds being "not strictly their role", I'm not entirely sure I agree with that statement. Assuming that there is in fact an agreed budget, I think legally it is the responsibility of the elected officials to oversea the expenditure of community funds; an independent administrator (i.e. not an owner in the community) may alleviate the "problem" to a certain extent, but ultimately any administrator is basically an employee of the community, and will have to take instructions from the president, rather than the other way round.
Losing, as you say, the co-president (and presumably continuing with just the one president) surely won't really change much? If these two are in cahoots, removing the official title from one of them is unlikely to make any significant difference in practice. I don't quite follow how this will give them less control.
Another point, regarding the minutes: if I remember rightly, for them to have any legal validity, they have to be recorded in a book of minutes and officially registered. A qualified adminstrator will be able to advise you on this and other aspects of community law etc. Why not try to speak to a couple, get some quotes, and pick their brains a bit on yor specific concerns and issues, and take it from there.
And finally, what do the other (four?) owners think about the current situation? Do you have any support for the kind of change that you hope for - or just the usual apathy and reluctance to get involved? Because if that is the case, I fear you may be orinando en el viento, so to speak.
I hope my idle thoughts on the matter may help a little.
Good luck.