Congrats on the bike, it's a good one.
Thought I would jump in here as I am a fan of Centurion bikes. I have a LeMans, an Elite RS and and 86 Ironman which is the same as the later Expert. Love the bike, love the ride.

For tires, I have had good luck with Serfas Seca. Inexpensive, good flat protection, and roll decent for an inexpensive tire, about $25 at REI. Ditto on the 700x25 for softer ride.