1. Average price for a average 17" tyre would be around $250
2. 18" on a euro is fine. You're allowed to go 2" wider than stock i think.
wider rims=harsher ride.
3. Yes, you do need to change your nuts. Using aftermarket rims require the use of tapered nuts. Lugnuts are normal nuts that can open unscrewed using a normal socket. Locknuts require the use of a special key that is specific to the locknut.
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