I know that having your own dump may be convenient, but unless you are planning on using it a lot the costs just don't justify keeping one for personal use. It appears you will be into this one for at least $4K and that is before your annual carrying costs of insurance, repairs, inspections, tags, etc.
IMO you will be money (and personal time) ahead to pay for material delivery and/or rent a dump for the few times you will need it. If you have your own rig all you are saving is the delivery cost, so if it costs $300.00 per load for the trucking cost of 20 tons of gravel, you could pay for 13 loads before you broke even with the purchase cost. And you still have the money in the bank earning interest.
For what its worth I use a small, 6x12, dump trailer and it can easily be towed by a 1/2 ton vehicle. And even that still costs $ to keep up. I just spent $300.00 for new brakes and $500.00 for new tires, Granted that will amortize over five years, but the point is that any vehicle will continue to cost real dollars to keep.