Hi Gavin,
I'm just at the same stage you're at... keen, but without the $4k budget
What Steve said is spot on. Just get a big, mean mount, like the CGEM II, or CGX, and plonk your C5 on it. Get a remote trigger/timer for your Canon DSLR, and go sick. You'll get all the light you need with long exposures, made possible by that solid mount. The wifi module and Starsense would be a big convenience to re-use.
Worry about another OTA next year. That C5 will bring enough to the DSLR to keep you gigling for a good while
