are the speed levers with the derailleurs compatible ?
So, as i understand, there are 2 options...
Option 1
*)to buy a 8-speed freewheel trying to fix it in the current wheel (what i must choose so as to be compatible with the shimano sis 8-speed (for the right lever and derailleur)?).
an 8 speed freewheel will work with thoseshifters and rear der. But you will have to buy also a new longer rear axle andrebuild and space the hub to accept this. This will make the wheel 130mm wide necessitatingthe re-spacing of your frame. Look on sheldon browns site regarding "coldsetting" http://www.sheldonbrown.com/frame-spacing.html 8 speedfreewheels were soon replaced by cassettes due to axle breakages since theunsupported area of the axles is longer than the 5, 6 and 7 speed wheels.cassette “freehub" wheels have bearings further outboard to support theaxles better.
and
option 2
*)to change the rear wheel with a shimano cassette compatible and to buy a 8-speed shimano cassette (what specifications must i see so as the wheel must have a small center-i think- to fix the cassette in the current frame?)
this will also necessitate re-spacingof the rear but will be more reliable in my opinion. Any road wheel of thecorrect diameter will be 130locknut'se locknuts and accept any 8, 9, or 10speed cassette.