I'm thinking of buying antique fountain pen, but the barrel is stuck to the nib and section because ink has dried it closed.
No. Dried ink is probably not why it's stuck together.
With vintage fountain pens, the most typical reason why the section is stuck to the barrel is that it's been glued to the barrel with resin. The best technique is to use a heat gun (or maybe a hair dryer), and to gently heat the area of the barrel/section until the resin softens enough for you to separate/unscrew the section from the barrel with your fingers. It doesn't take a lot of heat, so I'd say start slow and gradually test for sufficient softness in 10-15 second increments your first time, until you get a feel for it.
Do not do the Frank Dubiel "Da Book" thing of using an open flame. He was a labtech and knew how to control heat with a bunsen burner. I, as a once-inexperienced vintage fountain pen collector, can personally attest that you can melt pen barrels over a candle until they're like taffy. The glass-nibbed Japanese celluloid pens (e.g., Spors) can even combust. Please don't ask me how I know that. [facepalm].
The nib and feed may be stuck in the section due to dried ink. The nib and feed are always friction fit into the section because airflow is required for the capillary exchange of ink/air so the pen can write. And if regular fountain pen ink was used, it's just water-soluble dye, so should dissipate with water. But. If India ink was used by accident, the pen is more or less ruined, because any solvents that can dissolve the ink are likely to dissolve the plastic of the feed/section as well.
You do not want to soak the entire pen because you can get water into metal internals (such as the J-bar (press bar in your illustration) if it's a typical sac-filler) that will cause rust. And if anything is made of hard rubber, prolonged soaking in water can cause discoloration (black will typically become gray). I usually do an overnight soak of the nib, section and feed unit once it's removed from the barrel, and then knock the nib and feed out using a nib block, if I can't just pull them out with my fingers.
A nib block is basically just some solid object with a hole through it where the hole is small enough to securely keep the section from entering, but large enough for the nib and feed. You simply use a punch and lightly tap on the rear end of the feed to push it forward from the back end of section until it comes out.
I also feel I should state that most commonly your issues with a vintage pen and restoring it are going to be with replacing all the rubber bits that have probably fossilized or gone sticky/soft. So, you will probably want to have a link to the Pen Sac Company.
See also David Nishimura's article on Pen Repair Do's and Don'ts.