GPC widens powers, but I would hesitate to use the phrase "whatever they choose" (although it may seem like it at times!) The Local Government Association publishes a user-friendly guide on GPC, entitled "The General Power of Competence" which I recommend.
After the election, if the council no longer qualifies, it cannot adopt the GPC, so will revert to normal powers. The power of well being was, in broad terms, the forerunner of the GPC, so although the concept still exists (and I find it useful to refer to it in the council's deliberations) it has no legal standing.
Parish councils raise money through the annual precept, collected by the district council on their behalf. They may also charge for services (cemeteries being a typical example), however the level of charges should be appropriate and affordable and should be calculated to cover the cost of providing the service, rather than generating a profit. Many parish councils fund specific projects by applying for grants from local, regional, national and even international funders. They may also fund specific aspects of their work by public appeal (war memorials, defibrillators etc). There can only be one precept. The annual budget-setting process must include all of the council's proposed activities in a financial year and it is through this process that the precept is set.
Many services previously provided by county and district councils are being passed down the chain to the parish and town councils, who have the powers to operate these services or to establish or support a voluntary organisation to do so. The Localism Act, which applies to all councils, provides a framework for communities to take on the ownership and management of amenities such as village pubs, post offices, shops etc. This would normally be undertaken by a voluntary organisation established specifically for the purpose, but the parish council may provide financial support, if appropriate. There are specific rules relating to permanent trading activities for profit and it is extremely rare for a parish council to involve itself in such activities.
In the broadest terms, the role of the parish council is to respond to local issues, address local concerns and to meet the needs of the local community (the majority, not just those who shout loudest!) If they are not doing this, they may need a gentle reminder. I hope your Parish Meeting goes well.