I am wanting to change my door locks, the passenger side has had an attempted break in at some point so it's bust up, also the rear door opens with... Any key. Looking online though the rear door lock barrels seem to be the same small keys that the drivers and passengers doors have. On my 1991 200tdi however the rear door has a much larger key very similar to my ignition key. What barrel should I be searching for to replace it?

I've been looking to do the same, but my doors are a little newer. You should be able to replace the barrels with a full set, all keyed alike. I've seen a few vendors carrying different sets (3 barrels, 5 barrels, 2 barrels). Do a quick search through your favourite vendor.

I had a thought of keying my rear doors on the 110. Not sure if that would work. I know I'd at least have to change out the rear door handles. I've got three keys for mine.

SP has an ignition and door barrel set....
http://www.splandrovers.com/category_s/136.htm