If I buy the i5 3450 whats the cheapest motherboard that I can use with it.
Note: Do not intend to OC but if the cheaper motherboard means 15%+ performance loss then leave it.
What would the main differences be between this:
http://www.overclockers.co.uk/showproduct.php?prodid=MB-364-GI&groupid=701&catid=5&subcat=1990
and this:
http://www.overclockers.co.uk/showproduct.php?prodid=MB-386-GI&groupid=701&catid=5&subcat=1965
Z77 is an overclocking platform, unless you need the specific features on it then don't go for Z77, go H77 instead.
The CPU you've listed can't be overclocked by multiplier, so the motherboard you choose will have no effect on things.
If you can, go for a K CPU - a 2500K would be a far better choice than a 3450. Of course you'll need to go for a Z68, P67 or Z77 motherboard if you want to overclock a K CPU.