The usual disclaimer: I'm not an engineer, lawyer, contractor, or septic installer... this advice is worth what you paid for it :)
As Ecnerwal says, I'd guess the system itself should be fine. If you have doubts, septic pumping companies often offer a pump out (necessary for inspection) and inspection for a cost perhaps in the $300 to $400 range (highly dependent on your area). Assuming you are spending large sums for this lot, that's money potentially well spent as the seller is undoubtedly marking up the lot for the system and you could use any material defects to bargain hard. I'm talking about the tank here. I do think the leach field can be damaged by vegetation. You don't want to see a bunch of trees growing in it.
The fine points of the risk, IMHO, are related to engineering and permitting. I'd ask the following questions at your building department:
- Was the system permitted?
- If it was, for what size house? Most counties will require a certain size tank and leach field for a certain number of bedrooms or bathrooms.
- Does the originally permitted size still meet code? In many areas, septic permitting was very loose even 10 years ago but this is an area that has more regulation now.
Basically, I'd make sure that the building department is going to be ok with you using the old system for the general size of house you plan.
Next, the question of engineering. Not all locations work well with leach fields. I'd ask the following questions:
- What kind of system do your neighbors have? Leach field, raised bed, pump-out tanks, what. Perhaps the original system was not engineered for the site conditions.
- Try to find out what the water table is. If the neighbors have wells or basements, they may be able to tell you.
- Is there a percolation test with the city/county/etc for when the septic was installed? No perc test is not usually a good sign. You could have one performed with a backhoe but you'll need permission from the seller and that will cost you a bit.
- Is there any chance that your lot is seasonally wet (even if not wetlands), has a seasonally high water table, is in a riparian area or in a floodplain? These conditions would all make a standard system more challenging. For example, a system on a flat lot between a house and a lake may have high water.
These questions are important because a raised bed system can be a lot more than a standard system and usually requires engineering design work.
Another risk for a new system is rocky ground or steep slopes.
If it was perc'd, permitted, and engineered, and the local building code hasn't changed, then you might be in business. If it wasn't perc'd, permitted, engineered, etc I would be leary of paying anywhere near the 'retail' cost of such a system. You might as well get familiar with the local building department anyways since you will presumably be dealing with them a lot if you buy the lot.
In final, if it were me, I would do the due diligence required to determine the cost of installing a new system. I wouldn't want to find out the reason the project was abandoned was because the standard system won't work and the required engineered solution exceeds the value of the lot.