My experience has been that the min number of skymiles required to book an award flight is 25000 but it will very depending on dates on flight times. It has become a little easier in recent years to redeem skymiles.
You generally recieve 1 mile for every dollar you charge on your skymiles CC, they do have promotions now and them offering double or triple miles for select merchants, so it will all depend on how much you use the CC and earn miles. Points will expire if not used within a defined period of time, not sure what it is currently.
There are a couple of other perks such as not having to pay for checked bags if you have a Delta Skymiles CC.
Buying miles is good if you are a little short of the required points to book a flight, sometimes they will have offers to buy points and get a 25, 30 or 50 percent bonus, (buy 2000 and get 4000) and could be cheaper than paying the fare.
If you go to Delta's site you can enter the flight dates and select the option for Award ticket and it will tell you how many miles it is to book that flight.
there will be a small fee of $5 to complete the transaction, that you pay for separately.
If you have any problems you can always call them for help.