i like the name?
Rob's comments have put me on the spot but over the years I have built a few engine and in doing so found the things I like about the different types
First there are 3 engines built by Hornby they are the A4's and the 2 A3's the early one and the late one
Buying second hand is always a problem and you might get a good engine or a bad engine how they have been used and if enough oil or after care has been carried out to keep them in good order as these engines like the full size engine need more care before and after a run.
So any of the range that Hornby made are ok and will be a good buy if the seller can tell you that it's condition is good with no faults.
the A4's are a straight live steam engine the only point to watch are does it have the vacuum pipe on the front buffer beam and coupling hook other things to Look for are the handbrake control in the tender and water scoop handle this will also apply to the A3's plus on A3's check the front steps are not broken off just behind the front buffer beam.
if possible take a look at the front wheel boggy set sub frame base if it has had very little use the paint on the under side will be in good order if the engine has been run loads of time the paint here will be scratch down to the cast meatal base.
i think the best of the range from an engineering point of view is the last of the A3's this engine has a good oil control system and can be listed as the only A3 with a removable chimney but really nothing to choose from the range they are are all good fun and give hours of fun never two runs are the same so it will be your choice at the end of the day that matters and if you need it Hornby and J H can sort out any problems you might have.