Depends on if it's a flip or a buy and hold. There's nothing inherently wrong with a septic or cess pool as long as they aren't leaking. Potential buyers often find the idea of them "squicky" depending on the area. If everyone else has one it will be normal. If most have switched over to sewer your buyer will probably expect the home to switch over.
Put in a septic inspection in the contract. It cost me about $300 to drain and inspect a cess pool at one of our flips. It wasn't leaking but if it has been it would have been thousands to dig it up and remove and replace all the fill dirt. Even if not leaking it's a lot of $ to fill in with dirt to decommission in place. When replacing with sewer, depending on your jurisdiction, plan on about 5k to do the work and up to 5k in city and SDC fees. On the home we did the city was having a "sale" on fees if you held the house for 5 years or more so we made a deal with the new owners to pay for the decommission and hook up if they'd apply for the permits. Assuming they stay it'll be free to them and I saved 5k.
As far as buy and hold and assuming it's not leaking then you could just leave it and maintain it. Making sure your renters use the right kind of TP might be a bit of a challenge tho. Plan on $200-$300 every time you want to drain it. My dads property is on a septic and he's only drained it once in about 20 years but he puts enzymes down there and always buys the special TP.
