I'd only consider used for a DIY install myself. Its not hard to test the unit off the boat before you start, but the hours of labour can soon make a new one seem a better deal.
If you just want a heater that blows hot air then I would consider used and instal myself DIY. You can buy (say) a good used 2.2kw eberspacher for around £300 you will need to add a bit for extra exhaust and a skin fitting plus ducting if you need it.
As previously advised set it up and test it before installing. Plenty of advice on this forum.
Good luck.
