The boat looks pretty clean in the pictures and if it has all the upgrades it says it does it is probably worth the money. Catalina 25's usually go for around 5-10k depending on condition.
I would definetely stay in protected waters though.
How long you can stay on it is more a matter of personal preference. Some people can liveaboard a 25 footer. Most people think its a good size to spend a weekend on.