Your plans should work. In my view you should not need a booster with the two leading shoe front brakes, and there are significant issues fitting one to a 2a (the mudguard is modified to make room on S3). If it is a diesel you need either a vacuum pump or a venturi setup.
Brakes on 109s four cylinder remained the same through to the end of Series 3, except late Series 3 four cylinder had the same brakes as the six, that is, wider front brakes.
There were some minor differences through production, but I am pretty certain it is all interchangeable.
New backing plates are probably available, from any of the specialist suppliers. If not, it should be possible to find S/H ones. They do not wear or get damaged as a general rule.
John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
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