Here's an 8 speed IGH wheel for under $400: http://www.sheldonbrown.com/harris/wheelsint.html

I'm not sure what type of bike you're interested in but you can find bikes that only have gears in the rear (no front derailleur) for not too much money. After the initial break in period (after which the bike shop should make adjustments for free), they really aren't hard to maintain.

I guess I would be tempted to give yourself another couple of weeks to see how much difference improved fitness makes. Then you'll have a better idea if a single speed is going work for you or not. Whether or not you'd be happy with a 3 speed depends on how you like to ride. If tooling along at a relaxing pace is your style then a 3 speed could be right up your alley.