If he is able to make it work as well as Caster with all its features then I'll have no complaints; 1500 dollars is not a terribly large amount, especially if many people contribute.
Now then, the conditions.
Would he agree to receive the full payment once the product is complete and in satisfactory condition?
If bugs are discovered after the job is done, would he agree to fix them?
I bet people'd be reluctant to donate with no guarantee of success, and those who do donate would be bitter if the final product isn't useful.