Jmc, I wouldn't even think of changing O2's or cats. until the engine has been checked out. You'll be doing it again if you don't do that first. I know what you mean, when purchasing a "new to you" used vehicle. The Jeep dealer put the wrong oil in mine and it was ~18000 miles overdue a tune up. How many miles are on yours? If it's over ~75K you should probably do a complete 60K service on it, if you want to get, and stay on top of everything. Do a little at a time as some stuff is fairly spendy. Fluids/oils/filters alone run ~$300, if you purchase them.
The PCV Valve is easy and inexpensive. Just be careful not to break it off in the manifold, as some have done. Don't use excessive force, even though it is fairly difficult to turn. Install the valve with the hole facing the back. This is just my way of doing it, as otherwise (with hole facing front) it will provide a direct path from the valve to intake manifold. The Hemi's have probably the worst crankcase ventilation system of any modern vehicle, and the valve is a real cheapo too. IMO, a catch can is a must. I have one on both of my Hemi vehicles. Cost of the PCV Valve is ~$25.
Your in town mileage is about average, but your highway mileage is way off. There almost has to be something not working as it should. Even mine with the wrong oil and worn plugs got ~18 mpg, highway when I first bought it. Take a look into the intake plenum. Manually open throttle butterfly, look at the backside of it, and shine a light into the plenum to see if it is wet. The backside of the butterfly will probably be real bad, and if the PCV Valve is not doing its job, the plenum will probably be wet.
Again, if you have more than about 75K on it, the ignition wires are probably bad, or very close to being bad. If you have an ohmmeter, check the resistance of each, from end to end. Rule of thumb is no more than ~10000 ohms per foot, but they should be much less.....between ~10000-15000 ohms for a 3 foot long (which most approximately are) wire. If you have to replace them, consider purchasing the Taylor Shorty wires. I've had them on my Hemi LX for at least 6 years now, without issue. It also gets rid of the "rats' nest over the top of the engine.
A new MAP Sensor also runs ~$25 or so. You probably won't be able to find one that looks like the one that's in it now. They've superceded the part number and the new ones have a mounting tab on them. You'll probably have to cut the tab off and also a small part on the intake to get the new one to fit. You can do a search and find out how to do this.