will 14x6 100pcd wheels fit on a mk3 coupe cheers

yes

will the offset matter? as I'm not sure what it is

could do. depends how low car is. what wheels are they?

weller car is std height just now what height would you recomend for 14's

centre bore needs to be 57.1 you may need to enlarge your existing centre bore or use spigot rings if they are bigger than the centre bore of the hub. depends on offsets too, as long as its positive it shouldnt give you grief in 6 inch, if its a proper high offset tho they may need spacers to clear calipers, id say 60/40 or 60/60 would be about right, 10mm top caps for the front if you want a bit more stance