freewheel to cassette conversion

Hi,

I am planning to replace the rims on the bike (Ridgeback motion rapide) as they are worn out. So thinking of getting rims with freehub which are supposedly superior to freewheels.

The bike currently has Shimano MF-TZ21 7 speed freewheel (14/28), 28-48 chain rings, Shimano CN-HG40 chain, and...