Hi, fellas. I recently purchased a pre-owned Dahon Impulse D7 in Beijing at a nice price. Having shipped it to my residence, I found that overall the bike was in a good condition. However, after further scrutiny, there are some problems with the bike tho. And I would like to hear professional advice from all of you, as this is my first Crmo frame bike. I really want to treat it right. Thanks in advance for any advice!

First. there is a needle-sized spot of rust on the seat tube, which looks quite deep and doesn't look like a surface rust to me.

Second, an area of rust is found inside the steel frame when peeking thru the holes of the main tube. I have no clue whether this only surface rust or harmful ones. (Zoom in and you'll find something shimmering inside the tube. Creepy)

Is it necessary for me to find a bike mechanic shop to have it repaired and refurbished? I was told that the whole service would only cost me 218 CNY. ( equivalent to $30).
If not, what should be done and what can be done by myself?
Is there anything that I could apply to prevent it from rusting? (Boeshield T-9 isn’t sold in China, so buying it here would cost me an arm and a leg.)

Any suggestion is appreciated!!!