Slipping Transmission

[London380sl]

I'm considering buying a 1988 300CE with 111,000 miles on. Current owner has replaced the hed gasket and redone the valves guides. Car itself is in pretty good condition. The only issue I found was the transmission was slipping when shifting from 1st to 2nd ( or is it 2nd to 3rd in these cars?). Transmission would start shifting at 2500 rpm and rpms would climb to 3000 before engaging with about a second delay between the shift. I was being light on the gas at the time. There seemed to be less slippage if I was a little more enthuaistic on the gas. it shifted okay in the other gears and reverse. We checked the fluid level and it was a little low. Topped it up but it didn't cure the problem. The fluid was clear and it did not smell burnt.

Am I looking a potential rebuild here?