This is timely, as I am in the process of refurbishing the original-issue kit that I built back in the mid-'70s, as well as building the kit as reissued as a P-51K some years ago (essentially the same kit, but with some extra bits - Aeroproducts and uncuffed Hamilton Standard props, a new canopy, some PR bits and the vents over the auxiliary air intakes on the side of the cowling). Both will be finished as RAAF aircraft, using Southern Cross Mustangs as my reference.
It's a reasonable kit for its age, but crude when compared to later 1:24 Airfix offerings such as the Spitfire VIII/IX. There is a gaping hole where the undercarriage bay ought to be, and no spar to ensure that the wing dihedral can be built correctly. It is bedevilled by "working" parts which don't really work all that well. Cockpit detail is chunky, and the interiors of the main gear doors bear only a passing resemblance to the real thing. The main undercarriage legs, because they have been made to retract/extend, have been moulded in the in-flight position and need to be shortened for an aircraft on the ground. The flaps are moulded up, and will have to be cut out if you want them down (which was the most common configuration for aircraft on the ground). And don't, whatever you do, use the "rubber" tyres - they will melt the wheels beyond redemption in very short order. There are other small matters, but nothing that some basic modelling skill can't cure.
There is resin available to cure most of these problems. In particular, Model Monkey does a very nice undercarriage bay which fills the gaping hole really well, as well as setting the dihedral at the correct angle. They also do a swag of other detailing parts, all of which are useful in curing small faults in the kit. There are resin wheels available to address the kit's tyre problem. Nothing yet to deal with the undercarriage legs and doors, but we live in hope.
I had hoped that the new 1:24 release from Airfix for Telford would have been an upgrade of the kit - I've seen the latest "Vintage Classics" release, and the moulds are looking distinctly tired - but it wasn't to be. Maybe next year!