Get a Samsung SM951 AHCI, you can get a 512GB one. Haven't seen a 1TB yet.
Samsung 850 are a SATA-based SSD and while they do have M.2 variations, they are SATA ones and won't fit the connector on your laptop, which is a PCI-E 3.0 x4 M.2 connector.
What type of SSD do you currently have? a 2.5 inch one like the abovementioned 850 or a stick type M.2? There's a way to fit both types in that bay.
Yes, you will need a bracket for a M.2 SSD. aeolisio has made some nice ones and you can buy 'em off Shapeways here:
http://www.shapeways.com/shops/aeolisioIt's recommended to get some little heatsinks to put on top, as well. These heat up a lot. Be careful about the height, I think 6 millimetres was good.
EBay linkYou will need something like doublesided conductive tape to make the connection between the chips and the heatsinks.
3M has nice quality pads like these that come pre-cut.
EBay linkThe two on the left and right are the SM951 AHCI and NVMe.