PC BUILD - Anything i should change?

Yeah no problems.

The Radeon R9 280x is about two steps up on the graphics card level.
GTX760
R9 280
R9 280x and GTX 770 are about equal.

Crossfire is the technology used when you have two or more AMD Radeon (R9 270x, R9 280 etc) cards, but it's now used to infer the action of adding a second card. The same thing with Nvidia (GTX cards) is SLI.

The motherboard you originally chose doesn't have the ability to Crossfire or SLI, while the Gigabyte board I selected, or the Asrock Pro4 has Crossfire support (No SLI support but you wont need it if you go with an Radeon card). Even if you you don't think you want to add a second card now, you might change your mind down the road, so it's always nice to have the option.

Also note, in regards to Crossfire, both cards don't have to be the same brand. So if in a year and a half, you see someone selling a Gigabyte R9 280x for cheap you could put that in your rig, and it would work fine with your XFX R9 280x.

Now with regards to the PSU, the XFX 550W is perfectly fine for now, but if you did decide to Crossfire you would need a higher wattage power supply, probably around 800-850W. You could get this now, if you think this is the path you will follow, however you probably wouldn't lose too much money if you just decided to get a larger PSU later on and sold you current unit.

A lot of people think that a higher wattage power supply means they will use more power. The rating on the PSU is only it's potential draw, but it will only use what it needs to power the rig. Also PSU's tend to be most efficient around mid draw, so with your set up, an 800W would actually use less energy than a 550w (not much though).

Now with the SSD, as Sincreator mentioned, it won't improve the graphical performance of the computer, but it will make loading games and programs snappier. For example, in BF4 i'm usually one of the first three people into a map due to me loading the level off an SSD as opposed to a HDD, but other than loading faster, there is no real advantage.
Also some, mostly old, games display instructions on the loading screen. If you put these games on an SSD you never get time to read them, so put games like this on the HDD.

The Samsung EVO series is generally regarded as one of the best SSD's for the money, and satisfactory for what gamers need. There can be quite a lot of variance in read and write speeds and a lot of other mumbo-jumbo that I only very vaguely understand with SSD's, so unless you want to do your research, best to stick with the EVO.

I've linked a cheap 850w PSU and the Samsung EVO 120gb below for you to look at.
The power supply, I would probably put off for now given your budget and that you don't need it now. Besides, they will only get cheaper.
SSD's are awesome, and it's had to go back to HDD's once you get used to them. In some ways it's best to stay away from them because they are addictive. I'm now looking at getting a second one, because as new games come out I want to put them on the SSD, and it's a bit of a hassle moving games from my SSD to my HDD.

Power Supply: XFX 850W 80+ Bronze Certified ATX Power Supply (£79.99 @ Amazon UK)
Storage: Samsung 840 EVO 120GB 2.5" Solid State Drive (£55.49 @ Amazon UK)

The slightly more expensive 120gb EVO on pcpartpicker just comes with a SATA cable and 3.5" bracket and screws. You'll get SATA cables with the MoBo so don't worry about paying the extra 5 pounds.

I hope this helps with explaining everything, any more questions just let me know. I knew less than you when i put together my first rig and the people here at toms put me on the right track.

By the way, when you plug in your MoBo, remember it actually takes 2 power cables. If you forget the sneaky one in the top corner, everything will power on but nothing with happen. You'll freak out, then when someone tells you, you'll be really embarrassed. Trust me.