No doubt you realise that fully expanding the A1200 means putting it a tin box to even start competing with an A4000d. In its day those that wanted the power bought the Amiga 4000 because it had far superior expansion facilities. Only after Commodore went pop did folk start to expand the 1200s out of the case. A base 4000 is very easy to upgrade. I have 1200s in tin boxes along with 4000s and I prefer the 1200s kept in the plastic simply because the unit was never originally intended to be used this way, unlike the 4000 which was. I'm a bit of a purist at heart.
Saying that, I even have 4000d's which have been put in bigger boxes.
http://www.commodore-amiga-retro.com/amiga/amiga_scuzz383.htmscuzz
http://www.commodore-amiga-retro.com