Just curious as I've never built anything larger than 1/35 but is there a noticable difference between 1/16 and 1/20.
I'm looking at building a 1/20 Jeep but to add any accessories I'll have to use 1/16 items. Would you guys say this is a big deal?

A Jeep model would be about 4cm longer (21cm vs 17cm) in 1/16 than 1/20, so 123% of the size of a 1/20 model of one. I’d say that’s pretty noticeable, but up to you to decide for your purposes.

Thanks

Which leads me onto a question I have pondered for years.. Why on earth has no manufacturer made a 1/16 GPW or Willys??

To some extent it depends on what kind of acessories you want to add. A rifle would look very odd in the wrong scale, a driver could work since humans come in different sizes.
A wrench or other similar tool could work, they come in different sizes too.
If it is anything which has a single fixed size in real life then it will look oversize. The difference is 20% and that is a bit much ...
/ Robin