29ers are good. It is actually the same wheel size as 700c, it's how they differentiate between mountain and other. Most 29ers is the $500 range would have forks that are probably not the best for a 300lb guy. If you are going to do a majority of road riding the stock tires on a 29er will be off-road oriented.
I second the used bike market and older steel mountain bikes. They rock. If you are doing mostly road and light paths, I think Schwalbe Big Apple tires are the bomb. The 2.35" / 60c if the bike will fit them. Most mountain style bikes will.
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