On average, a gas engine is good for about 20,000 gallons if it doesn't have a blower on it. I think that works out to be about 2,000 hours with average usage. My understanding is that manifolds, elbows, and risers last a long time in fresh water versus 5-6 years in salt.
Not running an engine is bad also. Without knowing the year of the engine and how it was used, it is almost impossible to answer your specific question.
Nah. Hours are fine. Condition and maintenance are the key. For reference, I bought my boat with nearly 500 hours on it - primarily salt water - and I didn't bat an eye at the hours.
Besides, 350's are a dime-a-dozen. You can easily find a reman long block, with warranty, for under $2K.
I bought mine our boat last May with 384 hours on it with all the service records from the dealer it has been in both salt and fresh water Lake Washington to Puget Sound it now has about 750 hours on it and runs better now than when we bought it. Low hours means very little if it hasn't been maintained I'd buy a well maintained high hours boat over a low hour boat that has been neglected there built to be used not sit. Get a complete hull and engine survey with compression test, riser, manifolds, bellows and drive checked.
2005 420DB with AB 11 DLX Tender, Raymarine Electronics (2x12" MFDs) with Vesper AIS
Engines
Cummins 450Cs, 9KW Onan Generator, 40HP Yamaha for tender.
When I sold my 320 with 5.7s they had over 600hrs. They ran like a champ and I'd expect them to run the same for many more years. As Dennis said, maintenance and the way they where used is the key. If you buying privately ask PO how was he running them (what was the sweet spot RPMs he was using).