I hate to be repetitive, but if you questioning whether you should buy it because of funding, you buying something that will require more funding soon.
The dynamite kit is a nice kit as far as 21 conversions go. But I suggest you rethink it, leave the maxx as a small block and save up for a MT that comes from the factory with a 21.
The maxx is a good little truck and is designed for a smaller engine. You may be able to get a kit and use a 21, but as Vince said, the drivetrain will suffer. I know mine did.