different tyres use different compounds, treads, rubber, different strength sidewalls; so will react differently.
if you balance your axles with different tyres front and back you'll have 2 different grip characteristics. you run the risk of unnatural sliding. or spinning the car out.
theroetically if one tyre needed replacing you should do both on that axle as running different tread depths on the same axle can have the same effect.
if you want to do all 4 but money is an object then you have several options.
neil on the forum sells tyres and offers a range but most interestingly is a budget tyre (can't remember the brand for about £60 a corner.
alot of users on here run a mid range tyre like the falken 452s or the toyo proxies i think your looking at around £400-£450 for all 4.
if you want to head into ultra high performance tyres some members are running Vredestein ultrac ssantas meant to be very good (got rated second place by evo mag, first place was the Michelin pilot sports i think) £510
if money is more available then the tyre of choice is apparently the michelin pilot sport 2. but i think your looking at about £750
hope that helps