How long should it take a garage to fit a clutch? think mines on its way out and need to make sure i get the best possible price :)

A quick check shows it's about 4-5 hours and costs a not-insignificant £600-700. I recall having a new clutch on my Renault years ago and they had to remove most of the engine to do it

Should be quicker/cheaper on the Fiesta, there's good access around the bay.  Basically need to remove battery and airbox from on top, driveshafts from underneath, remove gearbox, swap the clutch, shove the gearbox back in and put everything else back.  With a ramp and air tools I'd say around 3 hours for a Fiesta Zetec engine.  May take longer if you've got any seized bolts.

Also i have been seeing clutch kits With a central slave cylinder and those without, which one do i buy???

With the slave cylinder.

If the slave fails in the future, it would require the gearbox to be removed again to replace it, and could contaminate the new clutch as well.  Not worth the risk of re-using the old one imo.

No where near £600-£700 on a mk6 fiesta 1.4

They are not bad to do on the fiesta, I'd say around a few hundred quid.