M-sport Connect has 18" and these look at the limit already:
http://www.m-sport.co.uk/m-sport-news/dovenby-hall/revealing-the-m-sport-ford-connect
Don't change the offset as it would ruin your wheel bearings prematurely (relatively speaking).
Any wheel with a matching bolt pattern and center bore will do:
https://www.wheel-size.com/size/ford/transit-connect/2017/