Bonjour!

I've recently come to the conclusion that I miss biking (last bike was stolen 2 years ago) and that motorcycling to work is neither fun nor financially justified anymore.
*motorcycles are fun, just not at 50kph in rush-hour traffic

So I'm looking at getting a bike for my commute to work and for the occasional casual ride around town (total tour is about 100km)

I'm a big lad at 6'5 and 230lbs, so I need a bike that'll be strong enough under my weight, and not tire me out too easily I did powerlifting for a few years, so I push out weights, but not so good with extended cardio.

I used to own a Marin mountain hardtail and used to commute to work on that sucker, but work was only 5km. I did a few longer distance tours on the bike (~50km) on bike paths around the city, but it wasn't too enjoyable. I got around 1000km on the bike over the course of 2 years before the bike was stolen.

Work is now 20km from my home and I'm looking for something a bit better to meet my needs. I'm currently looking at KHS bikes, mainly because my friend is a distributor and I pay far less that MSRP. I'm not deadset on KHS, but they do meet my financial criteria.

I'm looking to spend under $500 (cdn or usd - same as of today) for a bicycle.

Would you all recommend multiple gears, fixed gear or single gear? Personally, I'm leaning towards single gear for the light-weight and simplicity of it, and it allows me to coast, which I do really like. My commute is predominantly flat, although there are some fun downhills on the way home that I can take (if I'm lazy)

I've never ridden single gear or fixed gear, so I really don't know what I'm getting into, though.

Any thoughts or advice?