Samsung 850 PRO is probably the best SSD in the market right now. That 10 years warranty is also no joke, just to show how confident Samsung is towards its reliability, durability and performance

However you could still opt for cheaper models, like the Intel 530 240GB which is quite cheap at around RM400 ish.

As to whether you require 256GB or 512GB, I guess that depends on you yourself lol. For me I *think* 256GB should be enough for me but if you have tons and tons of programs and games, probably should go for 512GB straight

I think I can only find 1 seller with cheap price for Samsung, but the Pro is definitely high priced.

256GB or 512GB will depends on your usage, if you're going to install many games at once, choose the bigger brother, otherwise 256GB should suffice for most current titles (i.e. COD:AW, Ryse of Rome, MMO, some other programs...)
And not to forget that SSD's Trim also depends on the space provisioning, 15% for optimum performance, so max you can use is around 210GB (256GB) and 420GB (512GB) respectively.

1. Fresh install? Clone?
2. Have you installed any other running programs, try "msconfig" to check your startup.
Ok, that was plenty of left over, you're fine with 256GB, can install 2 more latest title...

If your laptop has disc drive, you can convert it to put your hard disk in, dual storage.
But if no, you can just buy USB3 SATA (not sure if hard to find one ) to plug it in as external.