all 3xxx series that are not 39xx are Ivy Bridge, the new ones.
In case you want to overclock, pick Sandy Bridge (2xxx) for sure.
If you're not going to overclock it:
2500K is still best performance/price and much lower temperature, but 3xxx series have lower power consumption, +5-9% performance, and support for higher ram speeds, native USB 3.0 and pci-e 3.0, much faster QuickSync, and 60-100% faster integrated graphics ( not that its important for you i guess ).
I think you should watch at other shops, because intel haven't stopped production of 2500K as of now, and they should still be in the shops.
For the motherboard, if you're getting the 3xxx series, check this table and pick one that has all features that you need:
If not, then same motherboard type as you originally planned.
All depends on your needs.
EDIT:
watched some specs, if your going with 3xxx series, i5-3550 is indeed a good deal, but it might be worth it going for i5-3570K, as its +100mhz and unlocked just for ~+15$. moderate overclock could still be aplied (+~300mhz) easily, if having a decent air cooler. As long as overclock didn't burn the cpu (impossible with these clocks), warranty shouldn't be voided, but need jalicano's word on this tho.
from old series 2500K remains best usually, but 100mhz for 10$ ( i5-2550K) is still worth considering, if you're out of the overclocking.