Road bikes are cheap compared to mountain bikes, so a 4-600 pound bike is a great price range.
Most people will say road bikes are more expensive, but I'm not entirely sure I believe either one is more expensive. For example, you can get a cheap (but solid brand) aluminum road bike frame on sale for 100 bucks right now on PricePoint.com. On the flip side, you can get a mountain bike frame on sale with the shock installed for ~500. Wow. What a difference, right?
Compare that to the components. An STI integrated brake/lever set in the "middle of the road" quality set on a road bike would run you 300 easy. Yet I could get brake levers, cables, cable housing, and middle of the road shifters for less than 90 bucks for my mountain bike...
To the OP:
If a bike comes as single speed, chances are it'll have to stay that way without some significant upgrades. A lot of single speed bikes that are built "SS" from the factory don't have cable eyelets mounted on the frame to hold the housing. Some do, but a lot don't. Secondly, a lot of single speed specific bikes from the factory do not come with a derailleur hanger, which holds the "gear shifter" by the rear wheel. Thirdly, a lot of single speed bikes come from the factory with the wheel "dished" for single speed usage.
Do yourself a favor and go look at any bike you have access to right now. Stand directly behind it with it propped up, completely vertical. Look at the rear wheel closely. Look at how the spokes are assembled to the wheel. You will notice the spokes on the drive side are almost not visible from the rear, while the non-drive side is easily visible. This wheel is "dished" so there's room on the drive side for the cassette - which is the "gear pack" that multi-gear bikes have. Single speed specific wheelsets typically don't have such a strong dish effect in the wheel. Instead, it's only slightly dished so it can accommodate one single gear, since that's all it's designed to do.
While I kind of showered you with FAQ's about how good an SS specific bike would work as multi geared, your basic and blunt answer (without a major overhaul) is simply... no. Don't bother. There are TONS of bikes out there. If you want a single speed, go for it. If a single speed is up your alley, great. I've had a single speed bike before, and I plan to build a single speed bike with existing bike parts this spring. But the reality is, for *me* (you may be different) a single speed bike doesn't belong as my #1 bike in the stable. I like speed. I like climbing. Therefore, a single speed isn't for me. If you like simplicity and you don't want to bother with extra gearing and whatnot, go for it. For me, single speeding was a LOT of fun at first - but once the honeymoon was over, I began to miss my climbing abilities and my bombing down hills. My single speed I plan to build this spring is of spare parts, and is intended for bumming around town. I'd be surprised if I'd ride it more than 40 miles a year once I get it done... But I know people who SWEAR by single speed bikes. You may be one of them. Just trying to play devil's advocate so you understand both worlds here.
But anyway, there are a ton of bikes out there. Many, many to choose from. Many brands. Many styles. Sizes. Colors. Geometries. Your choices don't have to be limited to one brand. Keep looking, be patient, and most importantly - ride the bike before you buy it. Even if you ride it in the shop and then buy online to save some cash - great. But riding it prior is important.
Good luck! Keep posting back with your updates - I'm anxious to hear what you end up finding.
