I live on Haleakala. I highly recommend that shop. Good bikes, nice people. He'll make sure you have good rides if you decide to go that route.
The race up is this weekend. You're two months late.
It's a great ride but keep in mind there are only a few places to stop for water. One at 3,500 ft and then not again until almost 8,000ft. There are elevation marks all the way up. If you follow the race route from Paia there are only 3 turns to make. The one above Makawao is maybe hard to spot the first time but the rest is pretty obvious -- just keep heading up. The only flatish section is after you make the turn above makawao as you head over to old haleakala highway. You'll want to start pretty much as early as you can since clouds and weather come in by noon. In october it will be very cold at the top. You probably won't notice it on the way up but if you plan on riding back down you'll definitely need warm clothes. At least a wind breaker and full gloves, etc. As far as the route goes, it's pretty steady. The middle section has some steep pitches but once you're in the national park it flattens out a bit. Don't forget to bring some money for the gate and they prefer that it is not soaked with sweat. They've mentioned that this is a problem with some cyclists.
While you're here riding out to hana and back is a fantastic ride. A group usually leaves from twin falls early in the morning on sundays. You want to avoid the hana traffic but if you go early it's beautiful although sometimes wet. Also riding around west maui can be really nice. It's only 60 miles but a fair amount of elevation change on the north end. Be careful in gulches as some stay permanently wet and there can be moss, clay, etc. that people don't anticipate.