Agree wheels are #1 upgrade....bought a set of used 700C wheels at local used bikes/parts store for not too much - around $90 included Shimano 600 hubs, Ambrosio rims, tires (which were junk), and a 6 speed cassette - slotted right in and worked fine with stock simplex. Another advantage is that the Mafac brakes typically have enough reach so you just need to adjust and go.

Mark