I agree with ketchup79 - get which ever you feel is within your budget on the day you choose to make the purchase. My only advice would be this: higher capacity almost always trumps more expensive models, as the added parallelism increases performance. But in general, any modern SSD is going to feel fast and responsive in day-to-day use. I'm still using a rather old Intel X25-m 80GB in my laptop, and it doesn't really feel any slower than the 250GB Samsung 840 PRO in my desktop. Just don't get the Samsung 840 EVO, as it's fundamentally broken.
Also, I've been researching PSUs for two upcoming PC builds, and I wholeheartedly second the choice of the Seasonic G-360. It seems like a perfect candidate for a low-power, silent PC, and getting 80+ Gold for that price (and in that wattage range at all) is amazing. Go for it.
Other than that, I'd consider trying a smaller form factor for the build - there is simply no reason in this day and age for an office/home PC to be that large. There are loads of great mini-ITX or mATX cases out there, and motherboards are also readily available. The BitFenix Prodigy is roughly the same price as your case (at least here in Norway), and perhaps half the size, but you'll need a mini ITX motherboard.
Both of the motherboards you list are mATX, so you'd can easily find a more compact case without looking for a replacement. The Fractal Design Define Mini should be a great option - the Define series has great soundproofing, and the Mini is a perfect shrink of the otherwise great R4. It also has a fan controller for up to 3 fans - great for making a quiet case - good airflow, easy to clean fan filters on all intakes, covers over unused fan openings (improves airflow and reduces dust build up, the latter is crucial if the computer isn't going to be maintained with any regularity), and fantastic overall build quality. It's marginally more expensive than your case, but totally worth it.
Other than that, it looks like a great build. The CPU is far more than you need, but that's good for longevity - with the G-360 running things and a well filtered case like the Define Mini, this should be a (nearly) zero maintenance build for home/office usage for years to come.