I think the crowdfunding needs to be clear about what the money is to be spent on, especially if it is going to a organisation that is not a not-for-profit. I don’t think it would be right to raise money for an undefined future project (although I do understand the wisdom behind doing this).
It may be worth building in a small contingency, just in case there are a few hundred quid of additional costs (beyond the fees to crowdfunder). If you didn’t spend all the money then Forest Hill Society could utilise the money in the short term for planting in the town centre, and there is no reason why future murals could not also be considered. But I wouldn’t make this a stretch target. When the project is successful, you would have a proven track-record to raise money again next year.
In terms of maintenance, I think you really need the graffiti-proof coating. It will reduce maintenance costs, makes it easier to clean, and actually helps to prevent staining from water flowing down the wall. It took about 15 years before the coating on the mural on the underpass wore away and moss, water damage and graffiti started to appear.