The wheels are the most significant change. Having started with the intention of keeping the 18 x 1 3/8 wheels, we actually found new tyres for them, which was not easy. But the rims were too far gone. Only 3 rims were salvageable. Plus we knew the braking in the wet with steel rims would be terrible. So we opted for 28 hole Sun Ringle CR-18 rims 20" (406) rims, which are of fabulous. The tyres are vintage bmx style Panaracer H406 20 x 1.75 gumwalls. They are superb. Unfortunately these tyres are not available in the 20 x 1 3/8 size or I would also put them on our Raleigh 20s. In hindsight, we should have opted for the 20" wheels straight away and saved time, cost and effort.
Note the maximum size and width tyre you can put on a Raleigh 18 is 20 x 1.75. You cannot go bigger. And, if you do opt for this size, which I recommend, then you cannot use the original mudguards. The original mudguards are too small to fit around the 20" (406) tyres. I thought about and tried using mudguards from a Raleigh 20 (20 x 1 3/8), but these are too big.