You will want your drain port as the lowest point in the loop. For me that meant adding a T to my intake on my pump. If you go my route, make sure you add an extra fitting for your drain tube that you will install when draining the loop. My drain tube is 18" long.
3603 tygon is correct I think.
What fittings are you planning on using? Dependent on the type, you may not see those fancy o rings.