A collector in The Netherlands just asked me some questions about Impalas so I thought I may as well share my answers generally here.
He was concerned about there being an all blue 248 (as mentioned in The Great Book of Corgi) and not sure what variations might be out there for the Impalas.
Although that book is a great resource for us collectors, the authors and photographers were a bit too close to the production lines and maybe not so much aware of what really made it to the shops. So there are entries which may have existed in the factory and even a few may have escaped into the wild but really there is little hope of you and I actually finding one.
Or being able to afford it even if we did!
So I tend to ignore some 'versions' and not worry about trying to find them.
That is a general comment - now to answer your Impala queries.
220 comes in pale blue and that interesting and very 1960s pink colour. The only variation is the blue one can have yellow or red interiors. Anything else is what I call an 'oddity'!
To the best of my knowledge, there is only the one 248 that was available to buy. That is the one with two shades of brown-yellow. I have never seen a variation of this but I can understand that, at the time of the change, there will have been examples made using the old colours.
248 is an example of the second type of Impala that was produced in 1965 in an attempt to extend the life of the old casting. They cleverly split the old one in two and inserted a piece of chrome all the way along, providing chrome grill and front end, stripes along the side and shiny rear lights and bumper!
This worked very well and they applied it to all the Impalas - the taxi, Fire Chief's car, Police car getting extended life with Type II models. There may, therefore, be examples of different tops and bottoms that factory workers may have decided to mess about with on a Friday afternoon when the boss wasn't looking! I haven't seen any, though, so I doubt if many actually made it to the shops. Some good restorers can also 'create' variants quite simply as all the parts match.
All Impalas have good suspension and interiors. The 220 types will have fixed wheels (fitted either way round) and there may be some with free spinning wheels (but I haven't seen any). All the 248 types have free spinning wheels which had become the norm by the time they appeared.
The thin roof supports can be easily bent and the screen unit extends a lot at the front which makes it vulnerable to damage. If the unit becomes a little loose it can hang down as there isn't much to support it.
My list of 'all models' may be worth referring to as I have compiled this over time with advice from many sources and been able to detect some of the innocent mistakes I find - even in respectable and well-informed sites. It is also constantly updated - for instance your question reminded me that I hadn't included the different interior colours so I have today updated that.