Fix one thing and another gives out.....

Have 82 240D Standard Transmission Sedan and have just repaired shifter/tranny on this car....

Fuel Level Gauge has been acting up....sometimes works...sometimes doesn't...and it sometimes only goes up to 3/4 full when tank is 100% full.....annoying as all get out, but I do have some indication of mileage and can read this on odometer to take a rough fix on diesel left in tank...but I could also forget to reset odometer and then find myself up the river.....

Checked out replacement parts online and new sending unit is $200-$300. And I'm told that using an old salvage unit is not a good idea in that these things are very tempermental to begin with....

Any thoughts on how to fix the part that I've got or is this a complete write-off now????? Thanks.