I visited KL for business back in the winter the week before TDL. I had a blast. Each day we were done by 3pm and I was out on the bike riding till 7:30pm (it gets dark very fast there).
If you are out of KL then yes there is one main way to Ghenting. It is a bit of a ride from town and the streets can be confusing but once you are on the may hwy you shouldn't have a problem.
To get there you'll have to ride with some heavy traffic at first. Don;t worry they are actually good drivers and since they are not used to seeing a cyclist ride so fast they're very courtious. In fact I had drivers cheering me on as I rode by.
Once you get to the main road to genting there are plenty of places to stop. Be sure to check the trees out when the climb starts as it will be common place to see Monkeys and some very exotic birds.
I am not sure what gear ratio you're planning to run but I rolled a 39x25 and it was fine. Mind you I spend a lot of time in the alps, the last 13km of the climb has some steep sections.
They only bike shops that I saw carried mtn bikes. They were not that high end. Depending on your currency back home, the exchange wasn't that great for Euros or CDN dollars.
You shouldn't have a problem with the bike however I stayed at the Sheraton Imperial and was footing the bill for a conference so of course they didn't say anything
TDL is very, very popular in KL. While I was there it was in the papers everyday and then had ample TV coverage.
Word to the wise, if you are trying to get around -- use the monorail, very convinient much easier then trying to deal with traffic. China town is a fabulous place to sample the local cuisine. I took 15 engineers out for a night on the town, they drank, ate as much as they wanted and I still only spent $300 bucks in china town.