I put a bit of oil in but it doesn't make much difference in my opinion, and your freewheel should be fine unless you've degreased it but you could just clean the inside and put some fresh grease on it if it ended up dirty
the standard disk brakes on onza's are pretty crap in my opinion, two shimano deore's and you wont think about your brakes again!
Also buy a isis bottom bracket as square taper usually do mess up.