The Seneca Five is an extremely capable aircraft. The engines are turbocharged and intercooled and wastegated so far superior to earlier Senecas.
Main differences are the climb capability which typically can be 1500fpm low and still showing 7-900fpm high level ie 18k.
They are fitted with oxygen and operate best high level where you will get a TAS of over 200 kts Low down expect 160 kts.
Most are certificated at 1999 kg which has an impact on landing fees and avoid Nav charges airways.
Handling is very docile stable and forgiving and the aircraft is an excellent instrument maching.
I always think of the five as a baby Kingair 90 and it will not be far behind.
Fuel look at 26 US gals per hour although you can go for a low cruise an get that back to 20 USG.
For maintenance costs may be worth Giving RGV at Gloucester a call.
With 2500 hrs on the five I see them as a trusted and much loved servant which handle ICE well.
I have flown them all over Europe! Summer winter night in attrocious weather so excuse me being biased towards them
Last point many talk of engine out ability of light twins over mountaiins the Seneca Five has an amazing single engine capability of being able to maintain over 16000 feet on one engine.
The older Seneca fives were fitted with King Autopilot which is far superior to newer Seneca Fives so dont be put off buying a 1998 model.
But do go for a Five as they are far superior in all ways to the earlier 1234s
Pace