However i have a friend who claims water gets underneath them easily and damages them, is this true?

Basically creates an area with new soil so she could plant things.

Would like to be able to do it without heavy machinery to compact soil before laying gravel.  

By 'perked' I am assuming you mean an engineer did a percolation test (how fast water seeps (drains) into the soil) to see where and what size septic tank needs to be installed for sewer waste.As stated in my first post, the tree situation depends on where you are building on the lot.

He is tired, would like to get out from underneath them, etc.. - but WHY is is wanting to move on - do NOT presume he is tired. 

They did tell me the original siding material they got didn’t fit, they were too thick to fit underneath the original siding, so they went with weathershield 2x contact boards and used dog ear wood for the small connecting pieces.

That is with re-using the existing ducts, but getting the ducts in the entire house cleaned out really well would cost maybe $300 extra, tops.I haven't ever had to deal with a house with a crawl space, but just like with a basement, you can make sure the gutters drain away from the house, and that the soil is graded to slope away from the house everywhere. 

I've noticed that some of the soil is starting to wash out from underneath my patio slab. 

Also the owner says that there is tons of great top soil in the back of the 7 acres that could possibly have potential (not making a decision because of these items).There may be some room for bumps in rent but I don't want to scare away any long term tenants and we ran the property both ways, actual and pro forma and it meets our credintials in both scenarios.Septic systems scare me also but they are a reality where we invest.

The wood laminate flooring panels are coming apart for a stretch of around 20ft, and in some places the gap between the panels is over 1/4" and through it you can see a corresponding crack in the slab underneath.