You need to assess your requirements as to speed and distance as well as what weight bike you can wrestle into your apartment (if you reside upstairs). A geared rear hub system might be optimal for you, but many have 135 mm rear axles and your bike might be 130. IMO, if it's aluminum, I wouldn't "stretch" the rear triangle. My first conversion was a 20 mph 36V, 10 w-h front hub from Dillenger ($679 plus shipping) on a steel mountain bike and it's still good for errands. The kit added about 15 pounds to the bike (17 pounds for the kit less two or so pounds for the parts which were replaced). Overall, the bike weighed as little as 40 pounds with a little bling, and 16-20 miles of range depending how much I pedaled. It's important to note with a hub system the rule of thumb is you need to be going about 50% of your top speed or too much energy is converted into heat. If DW responds, he'll mention similar (probably less expensive) kits from ebikeling and others have had success with superpedestrian, Hill Topper, Leeds and others; many are well below $1000 for the complete kit. If you're going to spend in the neighborhood of $2000, I would look at bikes from Specialized, Trek and/or other major companies and wait for a sale or new model year. These probably will be mid-drives, but satisfactory for your needs.