Sounds like you are going to get a deal on the house, Thats good.
The issue is how much money do you need to put in to make it worthwhile in your location. Im sure you know all this going in.
The irigation you talk about is called a curtain drain or swale around the home to direct water away.Also consider the gutters to have a seperate leader system directing water away.
The rot in the basement can not all be found by a home inspectionas they are visual only, No taking apart of any systems is standard since you dont actually own the home.
My opinion would be to get the inspection first to find any other problems, then you have all your defects listed and your to do list ready.Then you can prioritize you pro's and con's list.
If you need a qualified ,LICENCED (New this month in NY) Professional, Go to www.ASHI.com.:
Good luck on your home buying. If you get it we will all have a good time helping you out with your projects.