Before BangkokBikeRentals, there was no options at all for rentals in BKK... other than long term Thai/Thai scooter rentals... even Frankc only had old bikes...
SiamWheels isn't even worth considering, it is a ridiculously priced 'bike time share' club for rich Thais who want to show off 'their' new Harley or Ducati... other than that, your only hire options are outside of BKK where tourists go...
If you have time, you really should consider spending the $100 and fly to Chiang Mai...
As for clothes, I bring everything from home, because I don't have time to waste shopping for gear... the range is Thailand is limited, and expensive, compared to home (Australia) or online...
The only exceptions is cheap helmets... if you are happy to trust a helmet that would sell for $120 in Australia, the same helmet will be about $30 in Thailand - Index brand... but a quality, brand name, helmet will cost more in Thailand than it does in Australia...
Cheers,
Darryl