You'll probably hear this a lot but, if you are 18+ get Age Verified. You'll be able to access the classifieds for used guns and the AV retailer section for new guns.
Get AV'd here -
Is that a good airsoft gun? - it all depends on what brand you buy. Many companies make the Sig 55x - ICS, G&G, TM, JG
About 375 FPS is good? - Don't be overly concerned with FPS it is easy to change with a $15 spring. Max limits are usually - 350 FPS indoor and 400 FPS outdoor (With 0.2 gram BB's)
Good for indoor or outdoor game? - It depends how you want to play, Generally speaking, 552 is smaller and better for close quarters, the 551 is a mid sized assault rifle more suitable for both.
How long the battery able to last? - It depends on the battery you get and how trigger happy you are. The mAh rating on a battery determines it's longevity.
How much will be the good price? - It depends on the brand you buy. You can expect to pay $400-$600 for better brands and $250-$350 for lesser Sigs.
It's able to get the second hand? - It depends wether or not you get AV'd. The classifieds have many used AEG's for sale.
I own a ICS Sig 551 and I think it's a great gun. If you search You Tube and Google you'll find reviews on it and all other brands, here is an example - YouTube - Airsoft Gun Review of the ICS Sig 551 SWAT AEG
You will find the people here at ASC generally don't like to spoon feed people who make no effort to educate themselves. A lot of the time you can find the answer yourself if you search Google or search for older threads here on ASC. The FAQs are also a valuable source of information. People are more likely to answer your questions if you get AV'd and take a bit of time to put together a well thought out educated question.
Welcome to Airsoft!