if the frame is still in good shape, a rebuild would be a great way to bring the entire ride up to date.
The Ksyrium wheels will take an eleven speed cassette. You wont have any problem mounting 11 speed cassettes up to 32T. (if you go 34T plus you will likely need a washer for the freehub)
The 7800/6600 are great but the new stuff shifts so much nicer. Jumping up 2 Generation of components (7900,9000 - 6700/6800) you will notice a vast improvement on the components. The convex front chainring greatly improves the shifting. The new housing and PTFE cables are amazingly smooth. Pretty much every aspect is nicer except the finish. ( I am totally partial to silver components).
Coming from DA/Ultegra dont if you can go down to R7000.
If a new bike is not in the budget, an update will be under well 1K....if you go R7000 or 5800 you can like go under 500.00.
The biggest down side about updating older frame is you might be stuck riding 23mm tires.