For riding on pavement, I would definitely swap out the tires. This isn't a big deal -- $70 will get you a set of Gator Skins. You can get other tires cheaper, but I doubt it will benefit you in the long run. I run my cross bike on pavement 90 percent of the time too. During cross season I keep cross tires on it and remove the fenders, rack, bottle cages, mirror, etc. The rest of the year I run Gator Skins. On my commute at least, my cross bike seems to be about 1 mph slower with the cross tires, given the same perceived effort.
If you do put an 11-32 cassette on a cross bike, chances are you'll need to change the rear derailleur too to get everything working properly. I've done that on mine, although I prefer the road gears for most rides. Also no big deal, although these little things add up. You probably want a triple crankset for climbs if you're a relatively new cyclist. You can run a standard 50-39-30 crankset with a 12-27 cassette and get reasonably low gearing.
Regarding the Motobecane Fantom, which one are you getting for that price? A Fantom CX is only $550 new (you probably know that, having bought a Windsor), so it wouldn't be an amazing deal at $400 used. The Fantom Cross with Tiagra components would be pretty good at that price. Whatever you get, of course, make sure it's the right size.