High idle is likely the aux. idle stab. air valve that costs around $300, or the idle control unit at about the same cost.
The blue smoke could be valve-stem seals ~$1k, or worse -- full top-end around $4k. Fully rebuilt engines starting at $7.5k.
I bought my '86 for ~$3k more than the price you have, but it had 91 to 97% of the original compression and perfect otherwise.
In short, I would say it is worth it only if a leak-down/compression test can prove it is only the valve-stem seals and everything else is to your liking. There are still plenty of great SECs out there to choose from. The model in general is quite reliable and the problems you listed are common and not so expensive nor difficult to fix. Check out mbcoupes.com, if you haven't already. Cheers, Andrew Seidel.
85 500SEC Lorinser
83 300D Turbo