good luck with finding a good 2nd hand one, i would strongly recommend a new one, if your pally with your local yard ask them if they would buy you one trade, i would go for a Hotpot from ASAP supplies all pre assembled making install easy, otherwise search the net for any deals.
Your 2nd hand one will proberbly be half full of scale, the insulation will most likely be below par and its a lottery on how long a life it has left.
If you dont currently have an immersion then you will need to wire up the new one, is your shore power hook up up to speck? what is your breaker set up?