hi i've got two questions.

I have a limited knowledge about bikes as i'm learning, i'm trying to do up an old carlton touring bike 1979/80 5speed? or 10speed? (it has a 5 speed cassette but 10 options of gearing) for everyday use. i have just bought new wheels for it as the others were ruined but the rear wheel is a about 2mm too wide to fit in the drop outs, i can pop it in the drop outs relatively easily but i am concerned this might be bad for the frame, it's only ever so slightly too wide.

The other thing is that i need to get a new rear derailer for it as the other on is knackered after 30 years of use, it has a tooth missing on the cog. I've seen supposed 5 speed deraillers on ebay for about a fiver? is there a specific type i need? everything else is on the bike is original, apart from the wheels. I have also bought them with a cassette this time instead of a freewheel, will this make any difference to the derailleur?