Do your self a favour and look at zero turns with a Kawasaki or Vanguard ( Briggs and Stratton Commercial) engine. I would recommend a Ferris but there are plenty of others (nearly) as good out there either sit on, stander or walk behind which will give you a better cut in a fraction of the time...
If you have done that many hours on 10/20w30 oil I would stick to it, using synthetic now could cause a problem. I used to use SAE30 in the FH series Kawasaki's with out problems for hundreds of hours but the new ones are specifying semi synthetic, involves more cost and you can never be to sure...
My thoughts are, no, that's not a small amount and you won't have enough meat on the metal left. They are obviously not the correct ones, take them back with the old ones as pattern.
I fitted cleated tyres to a stander which were great when conditions were reasonable but when it turned really wet they were just to hard on the turf so I returned to turf tyres and learnt to drive accordingly with the winch as back up.