Look for rust anywhere on the motor - a decently maintained freshwater-only 05 should have VERY little external rust. Look for blue or black smoke under different loads. Don't mistake steam for smoke - smoke lingers - steam is white and disappears. Look at the fluids - black or smell burned? Tranny fluid s/b red, engine & Vdrive (if it's a Walter) should look somewhat like honey. Vdrive fluid will stink like sulphur (90W). No fluids s/b milky.
Run a compression test if you know how, or have someone do it for you. Spin the prop by hand with the boat out of the water and see if it binds.
More noise in reverse is normal. Quiet, fast clicking noises from the top of the motor are ok - fuel injectors firing. Louder, slower knocking is very bad - bearings, valvetrain, etc. Not very likely on such a new boat, though.
Can't think of anything else right now...