Well, the best way to tell if the siding needs to be replaced is: Is it cracked? Then it will need to be replaced. Is it soggy and really spongy/springy to the touch? Then it will need to be replaced. A little soft shouldn't be a problem but that is a hard call to make online without being able to inspect it myself. Also, if you are in an area where there are termites or carpenter ants, be sure to make an attempt to verify that they are pest free or you will have an even bigger expense on your hands.
When you look at investment property, if you are limited in your knowledge of construction costs in your area, the best rule of thumb is to try to find someone that you trust enough to look things over and give you a ballbark estimate to bring it up to speed and add another $5,000 - $20,000 to that figure.
If you don't have that option try to calculate what it will take to repair and then add another third to the price. So, if you believe it will take $50,000 to fix it up, you should keep in mind a figure of $75,000 at the minimum. Believe me, there are always things you miss. So, add on the high end figure to repair the property to the base purchase price and see if it is worth purchasing. Look at it as something that if you absolutely have to flip if you hit a personal financial snag, you want to make sure you will be able to get your money out it immediately if you have to.
Any other questions I can be of help with?