Check the lower intake manifold gasket for continuity if you can, if it reads OL then I'd highly recommend you replace the lower intake gasket with the FEL-PRO Part # MS98014T Aluminum framed gaskets, also if the upper intake can be cleaned up and reused, then you can get this UIM Kit, make sure to remove the EGR Stove Pipe and replace it with the reduced diameter one.
UIM Kit. DORMAN Part # 615207
If your upper intake has any softened plastic around the EGR port, or around where the throttle body gasket goes, then I'd consider replacing the upper intake manifold with one from IneedParts, it has a steel bushing where the EGR Stove Pipe passes through, so that with the reduced diameter stove pipe, and you should not have to worry about leaks, or having the engine hydra-lock on you when the old Nylon 6,6 LIM Gaskets fail, they are known to fail from around 60k and up, if that happens and you get coolant in your cylinders, then you can count on needing a new motor.
Here is the UIM Kit I recommend if you have to replace it.
At the mileage it is, I'd consider replacing the water pump, if you do I recommend the AC Delco one with an aluminum framed gasket, of course you could wait till it leaks and replace it then.
Check your EVAP hoses, and connectors, I'm betting all of the soft ones are dry-rotting by now, I had one under the right rear of my Park Ave that dry-rotted so bad it came off, and I was getting shudder under light acceleration from it.
I'd jack the front end up and tug on each front wheel from the 12 and 6 o'clock, and from the 3 and 9 o'clock positions, see if you hear any noise like clicking, at that mileage I bet the tie rods are starting to get play in them, you'll know if the tie rods are bad when you tug from the 3-9 o'clock, at 133k I had one inner tie rod that had totally failed, and the other starting to fail.
That, along with rotors that had a bit of lateral runout caused me to have a lot of shudder when turning, after fixing all of that I found that the struts are worn out and were adding to the shudder/wobble felt.
My ball joints bushings are getting dry-rotted at 133k, so I'd check yours as well, I'm betting they are starting to dry-rot if they were never replaced with grease-able ball joints.
As for your fuel gauge problem, for around 65.00 you can get a Dorman Fuel Sending Unit to replace it.
If all the other problems are not too pricey I'd guesstimate you'll be investing around 2,000.00, possibly up to 3,000.00 in parts in the long run.
In there I'm figuring for new plugs and wires, coolant, oil, and the maintenance basics it probably needs at the mileage it has on it.
The car is listed at around 3k in Kelly'* Blue Book, that is for good condition, I'd say with the current issues it has, and the money you'll need to put into it if you want it to last a while, are worth it due to how solid of a motor these 3800 II'* are once the UIM/LIM intake issues are taken care of.
How is the body overall?
If it is good then I'd get it, but try to see if they will cut a deal with you, offer 850.00 cash, I'm betting they'll take it, or counter offer at 900.00, if they do you save 100.00 to out towards the many repairs it will need.
Oh yea, if you want something to help keep the transmission holding on if there are any problems with things like the torque convertor causing shudder, then this stuff works really well to help add a little life to the transmission, note that if there are real transmission issues causing your shudder under load, then this would just be a band-aid..