A small reservoir is favourable, as it's ment to catch the air-bubbles in the loop. Most setups I see totally fail in this question tho, as the reservoir is placed in the wrong position. A reservoir needs to be placed on the highest (vertically speaking) place in the case to work like intended, as the air-bubbles automatically will find their way to the highest point in the loop... simple physics! So an ideal loop has the pump in the lowest spot of the loop and the reservoir on the highest spot in the loop with the rest of it's components (rads and blocks) inbetween the two. Vertical alignment is what we're still speaking about. A filter is not necessarily needed permanently in the loop actually, and I've allways used a simple stainless steel coffee-filter when installing the loop. Filtering upon filling the loop makes sure that you don't have dust or other small pieces left in the loop clogging the small structures of the waterblocks. The product from aqua-computer there is a nice feature tho, as it comes with ball-valves, letting you easily drain and refill the loop for example. Good position for it would be in the bottom of the case, between the reservoir and the pump, so that when you fill the loop through the reservoir it instantly catches all dust and small debris, before it get's pumped into the loop.