1. single-chainring geared bikes are nice commuters. I've got one myself. Albeit with a much less expensive crank. You could sell your Sugino 75 (people pay lots of money for those things) and get a much cheaper single-chainring crank. Or not.
2. you can get standard-width chainrings for 144 bcd. Likton's has
Sugino 42t and 53t rings in this BCD. I assume Harris has this sort of thing as well.
3. you may need to get a new rear wheel if you've currently got a singlespeed rear wheel. Or at least need to redish and respace the rear wheel to take a multi-gear freewheel
4. 7-speed freewheels require a fair bit of exposed axle between the bearings and the dropout, leading to bent axles for many riders, except those with special freewheel hubs (e.g., Phil or American Classic) or Campy hubs. Affects heavier riders mainly. If you're a lightweight, no problem. I weigh 180, and have set up my commuter with a 5-speed freewheel now.
5. Your current setup probably has chainline optimized at 42.5mm if set up for a singlespeed. This will also work about right for a freewheel on a 124mm (or therabouts)-spaced rear wheel.