Freewheel to cassette Conversion 2016 Windsor Wellington 2.0
Wheels with free hub bodies for 8 to 10 speed cassettes can accommodate a 7 speed cassette with a readily available spacer. 

Your bike almost certainly has standard road bike 130mm dropout spacing, but it’s a good idea to check before purchasing s new wheel. 

Derailleur should work fine, will probably need limits reset and indexing adjusting. 

It’s a good idea to get a new chain with a new cassette. A worn chain will accelerate wear on a cassette.