With an 8' dish, I wouldn't spend too much to optimize the feedhorns. The reflector size will ultimately be the limiting factor for what you can receive. You likely won't see a significant difference between a Corotor and a combo LNBF with most widely viewed transponders. For example, you likely won't be receiving the 97w 16APSK C-band mux of popular channels with an 8 foot dish regardless of using either feedhorn type. The KU reception for either type of feedhorn will be similar to what you would expect from a 1-1.2m dish. For the difference in cost, I would certainly consider the CK1s for an 8 foot dish.
With that said, combo C/KU LNBFs are a compromise over a servo polarity controlled feedhorn. I prefer a servo to optimize the polarity for each satellite and each polarity. Also the LNBF design is optimized for C-band reception and KU reception is highly dependent on the dish design. A combination of the reflector's FD and FL will present the KU signal in phase, but on other reflectors the FD/FL combination will present the KU signals with phasing issues.
No deals on new Corotors, but no reason to not buy used. The difference on the total cost will be based on your choice of LNBs. Basic +/-3MHz stability DRO type LNBs can be purchased for under $100. +/-5KHz PLL type LNBs can be several hundred each.
I also would buy at least a 24" actuator. Am a big supporter of the Venture brand and would suggest the extra money spent on the maxi ball version is well worth it!