Hi an welcome to CS.
Second hand commercials are great..... a few things to look out for.
Draw some water from the boiler hot water outlet..... let it cool and then smell and taste it..... does it have a funny smell and an off taste?- if so milk has been drawn back into the boiler and has tainted the water - a full strip and clean required - very expensive unless you can do it yourself!!
When extracting is the pump quiet and the pressure remains constant at about 9 bars (doesnt fluctuate wildly on the needle). If noisy and /or fluctuating pressure is observed - pump is on the way out - again an expensive repair.
If it is still connected - try it and taste test the resulting coffee.... check the steam production - it will be lots!!!
See how much power it requires - you may need to modify it so that it can run on a less powerful mains outlet or have a special 20 Amp run installed direct form your power board.
Does it have to be plumbed in (La Cimbali and some other brands need positive mains pressure)...... not a problem is you do intend to plumb it in..... but you will damage the pump if you try to run one of these machines from a reservoir - if it has a procon pump.... not a problem.
Has it been run on a water softener.... if not it might be full of scale.... not too bad in Melbourne (their water is very soft) but a real problem in Perth and Adelaide where you can chew the water (also many country towns around Australia). Descaling is also expensive - unless you can do it yourself.
And above all - good luck!
And one last thing..... make sure it is a model which CoffeeParts carry spares for (or an Boema where spares are available direct from the manufacturer here).... Pedro has an excellent range of spares for the machines which they supply them for.... he is also extremely helpful and knowledgeable about these machines. You will need spares at some time (The machine I purchased had also been "regularly serviced"..... but not all service organisations are the same >:()