Hmm... if there are plans to upgrade to the K CPU then you should have an OC'ing mobo then. If not, the price is what matters then. That's where I fail
. But generally speaking, if it is within the budget and/or the price difference is not worth the trouble, go with the OC-able mobo.
However, I heard MSI uses a technology, called MCE if my memory serves me right, that runs the CPU at its highest boost speed even when all cores are used (normally when all cores are used, the speed runs at the base only) on some of their mobos. I don't know much about it so you may want to look it up. It could help with the non-K CPU you choose and at the same time makes use of the cooler you already purchased since boost increases temperature.
Even non-K CPU's can be OC'ed, by the way. It is just that they are not designed for that. Otherwise how can Turbo Boost work on them.
I guess there is nothing to do if the cooler is already purchased and cannot be returned anyway. Keep in mind that the cooler might not be compatible with future CPU sockets. If the cooler comes with different socket brackets, take good care of them too. I used one professional cooler with two generations of CPU's thanks to that.