Litecoin Rig Help

[GeTecknics]

so i wanna get into litecoin mining i have a problem though i can only have one gpu since its a mini itx mobo

other specs are:

i3 3240

8gb ram

500w psu

p8h77-i 

i know the ram and cpu are overkill but it was my old lan rig that i dont have need for anymore so i want to turn it into a mining machine

 

So on to my question what is the best gpu for under $180 for mining 

i've seen 7850's for as low as $95 on ebay is that viable for the price and will i see a return using it 

any help is appreciated thanks 

 

also what would be a good os as i am about to run out on the trail version of win 8.1

[CoolBeans]

7870GHZ