You can do a search on 300E issues. I will take the liberty of summarizing by saying that you should pass if the climate control and AC do not work perfectly. This can be an endless money pit.
The ABS and traction control are also problem areas. Outside of that, look for good records and general mechanical condition.
They are great cars. I truly enjoy my '86.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe