To keep the same overall size, 18" tires will all have a much shorter sidewall (less 'cush') than tires for a 16" wheel - Bigger wheel, less room for sidewall. Our 55's on 16" wheels already have a pretty short sidewall
Our 55's (55% of the 215 width) have a 4.66" sidewall, the 45's (45% of the 215) have a 3.81" sidewall - 18% shorter
The 215/45R18's are a close match to the stock 215/55R16's - Only about 1% too big. Probably the best match you'll find, but you won't like the sidewall height if you want more cush
https://www.tacomaworld.com/tirecalc?tires=215-55r16-215-45r18
Also, the BIG problem will be finding a tire with the proper load rating - Don't think you can. Our stock 16 Continentals load rating is 97W and the best of the 215/45R18's that I see are only rated 93W - Meant for a car and not a van . . . . especially not a loaded van. If you present this scenario to one of the experts at Tire Rack, they wouldn't sell you a 215/45R18 to put on a Transit Connect
The 235/45R18's you asked about in the other thread would be a much better choice for you, even though they are too big around - A 215 tire is pretty narrow to stretch onto an 8" wheel and with the 235's, you can get a properly load rated tire - There are several rated 98W, so make sure you choose one of those. The sidewalls will still be shorter than with the stock 16's, but not as much - 45% of 235 is taller than 45% of 215 . . . . but still less than 55% of 215
Don