The only thing to look out for is that with a lot of mileage the heating elements in the smoke unit can carbonise the oil used for the smoke. The carbon tends to block the channels to the chimney. They're not too difficult to clean though and dismantling the A4 is very straightforward.
I refurbed one of Dapol's ex-demo models for Brian Fardoe when they first came out. I suspect yours won't do they track time that one did though!
