My 01 altima just crossed 100000 miles and the mechanic suggested that I should not only change the spark plugs but the plug wiring too and gave me estimate of $400. Is it really necessary to change the wirings?

What other necessary servicing should I get done for 100000 miles?

New plug wires is probably a good idea, but that is WAY WAY WAY overpriced. Check online parts stores to get a better idea. Find a new mechanic.