To begin with, you are considering a very good marine diesel engine.
You can buy the engine locally to you @ any Cummins Marine dealer certified in Quantum QSB sales and service. I'm thinking all are.
You could "shop the internet" for the best "street price" but I'd advise against that unless you feel comfortable asking the local Cummins guy to come do warranty work on an engine you bought on the web!
The QSB is a very reliable robust marine diesel BUT it is a TIER II electronically controlled engine. Fault codes and the rest requiring a
proprietary (Cummins) software package etc. etc. ect. should something go wrong.
I'd buy it from a LOCAL dealer to you who has the very good service capability record. Try to buy it from the dealer that sells the MOST engines to COMMERCIAL customers. Ask around.
Any engine you buy, get it installed properly paying particular attention to the exhaust setup and install.
After that, run it, take care of the sea water side of the cooling system, keep it a wisker under propped and have fun.