Our team has enough money to buy Belmore Sports Ground off the council
You mean the Leagues Club? The "team" doesn't.
In any case, money doesn't mean much if the Council isn't selling.
And, even if they did sell, there are planning permissions, transport & infrastructure issues, adjacent residences to consider (acquire, ignore, or placate), all before you get to the costs of designing, funding, and building a complete stadium that meets the myriad of needs (now & future).
On top of that, we have already sunk plenty of money (ours, but mostly public) into upgrades of admin & facilities at the current site. Not sure the donors would look too favourably on their demolition so soon after implementation; especially if you are seeking additional funds for the new project from the very same donors.
If an upgrade of Belmore as a minimum NRL stadium was ever feasible, it needed to begin two decades ago, with strategic acquisition of surrounding properties as they became available, and anticipation of public infrastructure works (like the Bankstown Line replacement), so that it was incorporated into the design (eg a Belmore Stadium station underneath a Grand stand, and attached to a Commercial precinct).
Would have loved for this to have been the case, but it's not and it wasn't.
Interestingly, the Club had taken some strategic steps...at Liverpool (acquiring properties, etc) but dealing with Council (a corrupt one) proved why these kinds of plans are fraught with challenges that are not always within the Club's control.
We are stuck at Stadium Australia for at least the next decade, and that's if we made a strategic decision now, and started actively pursuing it now, for a long term alternative. With our economy in an extended recession/depression, there will be little chance of the public funding that went to the Eels, Rorters & Panthers for their home ground re-builds.