18 lots on a well and a septic field, $240k, rural county. Is this a starter or a trap?

A well and a shared septic aren't automatically a trap. They're a business you'd be operating, and you'd be the water utility for 15 households whether or not you think of yourself that way. So the questions are all answerable ones.

For the well: get the driller's log if it exists, have the water tested, and get a pump and pressure tank inspection with an age on both. A well serving multiple households often falls under state public water system rules once it crosses a certain number of connections or people served, which brings sampling and reporting obligations. That threshold is set by state and sometimes county health rules, so call the state drinking water program and the county health department and ask them directly what an 18-lot park with one well is classified as. They'll tell you, and they'll tell you if this one is already in their system or has been operating outside it, which is a very different purchase.

For the septic: find out the design capacity and the age of the field. A field sized for 1972 and carrying 15 homes is a different asset than one with headroom. Get it pumped and inspected as a condition, on your dime if you have to.

At $240k for 15 paying lots you have room in the price to be wrong about a pump. You don't have room to be wrong about a drain field replacement plus a county consent order.

So yes, spend the inspection money. And do the two phone calls first, because if the health department says this well has never been permitted as a public system, that changes the price rather than the answer.

At this size the other thing to sort out early is who lends on it. A $240k park with a private well is small for most park lenders and odd for most local banks, so you may be looking at seller financing or cash. Worth knowing before you're $4k into inspections. Ask the seller whether he'd carry, since a 78-year-old mowing his own park sometimes prefers payments to a check.

Somebody has to mow it after he stops. Fifteen lots won't support a payroll, so it's you, or it's a local guy at whatever he charges, and that line item is a real percentage of $215 x 15. Not a reason to pass. It's a reason to build the expense sheet from scratch instead of from his notebook, because his labor cost has been zero.