is the 603 engine a sleeved block?or does a rebuild require boring and new oversize pistons?

Yes, the 603 is sleeved and crazy expensive to rebuild.

are the parts more expensive than normal or is it the labour involved?

About $1500 for the parts if using oversize pistons and boring for them. That is mostly for just the pistons and rings. About $3k is a good figure to use with the associated machine shop labor and the "other" parts. Maybe $4K if you are being thorough. The cost of pistons alone will set you back over a grand.
A resleave will cost more. Would still need to be bored to match the piston tolerances. Hard to get exited about reusing old pistons as they could too easily give out and you would have to start all over again.

603 Pistons are about $300 each (need 6) and the sleeves are about $70 each...
vs.
The 617.95x engine pistons are about $200 ea (need 5) and the sleeves are about $35 each.

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Over 21 years I owned several:
w108
w110
w111
w115
w116
w123
w124
w126
w212

Question: If you have the crankshaft turned, does this not make weaker bearing surfaces as the original heat treated bearing surface is removed from the turning?

If so, would it be worth renewing cylinder sleeves?