Congratulation on your new guitar! And a belated welcome to Chordie.
From the Yamaha Guitar Archive (can be found via Google)
Year(s) Sold: 1993-99
Original MSRP(US$): $449.00
Top: Solid Spruce
Back / Sides: Nato
Color(s): Natural, Violin Sunburst
Your serial number decodes as follows:
7 = 1997
11 = November
20 = either November 20th or the 20th day of production that month. I think it is the former.
519 = the 519th unit of production of that model per some unit of time.
And for the record, like many manufacturers, an "s" suffix means solid top. You've done well.
As for strings, I'd use what ever sounds good to you. I've owned several Yamahas and I tend to use lights on all my guitars.
I've owned a 1979 CJ-818, 1973 FG-260 (12 string), and an SJ-180 (1981-1985, can't read the serial number). All are sturdy, well-built and playable and, in my case, all laminate construction (no solid tops). Although the old 12-stringer had succumbed to the tension of the strings and the action was a little high.
There is a cultish following of Yamahas so you should be able to find as much information as you are willing to dig for. I found myself running down many a rabbit hole but I can identify many of the Yamahas (to the model) by the grainy photographs posted on Craig's List. Two days ago I did just that. I called about one listing and asked the guy if it was an SJ-180. His answer: I don't know, let me check! I was right. I am also baffled that someone can own an instrument for 25 years and not know the model number, specs, etc. I guess it's my engineering training that requires me to know these things.
Happy playing. Any Chordie member will tell you it is customary to post photographs of your new guitar. For that you will have to reach to an expert, I still haven't conquered this process.
p.s. the optimum place for this type of post is in the "Guitars and Accessories" forum