Although I think a 300 watt power supply would be fine for this system, I see a potential problem with that particular Seasonic unit you have listed. A standard SFX/Micro-ATX power supply's dimentions are approrximately 4.9" wide x 2.5" high x 3.9 deep. Due to the way the fan housing is situated on that 300w Seasonic, the height is actually 3.0". So, that is an extra .5" of space you will lose in an already tight enclosure. Looking at the pictures of the case you have, it looks like that extra .5" may interfer with the motherboard rear I/O plate section.
The original Silverstone you picked is a standard size and much nicer. It is 80-plus bronze certified, has a robust +12v single rail, can be converted to a standard ATX opening via the included adaptor plate and has just about any connector you would ever need.
That said, The Silverstone has its flaws for such a small case. The sheer amount of connectors will result in a lot of unused wires that would need to find a home. Take that and factor in that the wires are sleeved, it will make for one heck of a time getting them all to fit (anyone who worked with sleeved wires knows how stiff they can be).
Perhaps you should look into this particular Seasonic power supply. It has the standard dimentions, not as many wires as the Silverstone and those wires are not sleeved (making them easy to minipulate). It also has an extra 50 watts over the one you currently have listed. The price is also a little bit cheaper and the reviews seam to be good (20 out of 21 or 98% giving 4-5 eggs with the one 3-egg review given by someone who didn't even purchase it).
SeaSonic SS-350SFE 350W SFX12V V3.1 80 PLUS Certified Active PFC Power Supply - OEM $44.99
Just something to consider,