How are things in Rio? (I have a female friend who lives in Rio, she would like for me to come down there and visit her sometime).
The R-50 was first manufactured in 1979, and was made until '80/'81. Its rated 45 Watts rms, with 0.09% thd. Sansui also made the R-70 (65 Watts), and R-30 (25 Watts). Similarly, the correspoding integrated amps would be the A-80, A-60, and A-40.
They are basically decent amplifiers, though they were marketed to the buyer as an inexpensively priced package consisting of an entire system, in other words, they were meant to be used along with matching components comprised of a turntable, cassette deck, a rack to put them in, and in some cases, a particular rack was offered with a specific set of recommended Sansui speakers (as shown in various Sansui brochures and catalogs).
For anyone interested in spending a bit more money for better audio performance, Sansui was making the last generation of G series receivers at the time, the G-9700....G-4700 models, which then changed in '80/'81 to the Z models, 9900Z...3900Z.
$200 seems a bit expensive in my opinion for an R model, but I realize that the selection of available Sansui models in Brazil is probably somewhat limited, thus you might have to pay a higher than average price. Hopefully, the amp will be to your liking and you are happy with your purchase. I wish you good luck with it.
ps: if you are curious to get an idea of what an A-80 looks like you can see one thats currently on Ebay, though this particular one is only in mediocre condition: