Yea get it good and warm on the hose and suck out the old oil. Just eyeball the level. The way this system works it doesn't have to be absolutely level, since you are just sucking oil out of the oil tank and not the rest of the engine. If you can get 2.5 quarts out you are doing good. I suspect (but have not tested it myself) that you may be able to get a little more out with the nose of the ski slightly down. Try to get your oil extraction hose down the holes in this photo, but be careful on the rectangular holes they have sharp edges that can cut the hose.
I also like Amsoil. I run their 4 stroke marine oil in mine. Be sure to run 10W-40 if you go with Amsoil. As you know Amsoil is more expensive. I've heard good things about Quicksilver marine oil. I'd say anything you find with an FC-W rating will be fine. Of course you cannot go wrong with Yamalube.
Replacing the whole speedo sender unit should not be too hard, except for the wiring. I've replaced the wheel, but never the entire unit. I don't know how the wire attaches to the unit, but hopefully an easy couple of screws. Should be a 5 minute job.