l wanted to know if l buy 17 inch alloy rim if they will fit intoa 206 peugeot, and are there any modifications that l may need to do in order to fit them

Does not the 206 normally have a 14'' rim? If u add 17'' rims, then u will have to add the narrowest tyre possible. Such as 205 / 40 R 17. There is not much room in the wheel arch for MORE rubber.

Consider what u must do if u ever have a flat tyre. Three 17'' wheels with skinny tyres should equal one of ur old 14'' tyres for a spare, AND CARRY the original lug bolts with u for that spare. The 17'' rims will have fancy longer lug bolts, yes?

l think u will have 600 quid of tyre/rim on the car. You'd better get a security lug set for this tempting booty for the thieves.

Finally, check this website for changing ur front wheel alignment (and possibly the 'ride-height') when the tyres r so different from standard..
http://peugeotlogic.com/workshop/wshtml/mechanic/206susp/206align/align206.htm