When overclocking, you should know that more heat is produced from the CPU. Therefore in order to attain a better overclock, you must cool the CPU more efficiently than the stock heatsink/fan if you desire a better overclock. You could look into aftermarket coolers, especially watercooling if you want to go the further mile.
As for the differences in the motherboard chipsets, Z87 only supports Haswell. Z97 supports the future Broadwell processors, whilst also retaining Haswell support. If you want to be "future-proof," you should get the Z97.
And for the record, Broadwell is NOT soldered to any motherboard; just like Sandy/Ivy Bridge/Haswell are not soldered. The CPUs will still be sold seperately from the motherboards, just like they've done the past decades; even going dating past the Pentium 4's.
Only OEM manufacturers solder CPUs into sockets, and it's usually laptop manufacturers with BGA sockets.
All the best.