If you are faced with approx $5k worth of repairs: sell it.
Get whatever you can for it. Go find another one. Find one that has records, a history of someone repairing as it needed things.
Our '91 300E was this kind of car. All books and records. Yes I have put money into it but the car is and was well worth it.
There are a ton of good 300Es around. Find a good one and you will be really glad you did.
My 2 cents,
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)