124 300E auto trans

Just acquired an 86 300E with 257K miles and one enormous problem. Expecting the worst I didn't pay much for it.
When it's cold the automatic transmission will only shift as high as 3rd. When it's warm It will only shift into 2nd, and I have to accelerate to over 35 and back off on the pedal before that will happen. Trans fluid level looks good on the dip stick. The pressure cable is attached at both ends and seems properly adjusted. There's no engagement delay when I pull the lever into drive. The shifts are quick and secure when they do happen. Will I get lucky with a fluid and filter change? Are there any other things I should investigate? What's a fair price for a rebuild these days? Thanks!
Pete