Ive got the s27 - same model range with pretty much identical internals.
The differences are that the s27 has a rotary pump and needs to be plumbed in/run off a tank and it uses a touchpad for dosing. I think the s24 is vibe pump and internal reservoir.
As far as I know - the Epoca range is very similar to the S-series and the only real difference is in the outer casing - plastic rather then metal.
The machine is built like a tank - with easy access to the internals and plenty of room to work in there. All parts are industrial quality - gicar pressurestat, copper lines, nickel-coated copper boiler etc. It also steams like a demon at the 1.1 bar Ive got mine set at. Parts and service are freely available from Mocopan - the Rancilio agent - if you are in Melbourne.
Downsides - no E61 head - so distribution of the coffee must be spot on or you get serious channelling (may not be as much of a problem on the s24). Heating of the group is via conduction with the same group setup as Silvia - which may not be a stable as a thermosyphon loop. Not especially pretty - but Ive gotten used to the looks of mine.
As to where you may find one - I got mine at a catering auction. Mocopan has the odd one through as well. Have you looked at a single group Epoca? They are newer and there may be a few more around.
Of course - try the sponsors for other options like Bezzera, Giotto etc if youre not set on a Rancilio.