It is hot yes,but I guess that is relative;if you are used to hot weather then it will be nothing you cannot cope with.If you aren't then just take your time,drink plenty of fluids and you'll be fine.You can expect it to be c.30-32oC most of the time with odd days a tad higher.
August in Istanbul is no busier than any other time these days.many choose this period for coastal holidays so foreign tourism doesn't increase especially and many local people will be away on their summer holidays too.
Flying is the quickest way to reach Cappadocia(I guess you're not interested in a 10 hour overnight bus ride...right??).Flights go from both of Istanbul's airports to Nevşehir and Kayseri.Flights to the latter tend to be far more frequent and cheaper too.Transfer time from there is not very much more than from Nevşehir,maybe an extra 20 minutes is all.
If you definately know you'll go to Cappadocia then it would be wise to book your flights asap.The nearer the date so the prices tend to rise and seat availability drops.
Turkish Airlines,Pegasus,Anadolujet and Onurair all cover the routes so you can look at their web sites and book directly with them.