This is a known issue for w124's but I believe it applies for w126 as well. It's most likely the reverse clutch pack friction disc that are worn. Just search "reverse transmission" in the titles only and you will get a boat load of hits. Your mechanic is right about how long you can go before it goes belly up. It will get worse...your delay will go from 1 sec to 4-5 sec...then slipping in reverse when you gas it a little or reverse engaging on an incline with the back facing up the hill. Some experience out right reverse failure too with minimal syptoms too. You also may hear some clicking when you are in reverse at some point.

You'll get some advice to do a partial rebuild of just the B3 reverse friction disc (800-1200) or overhaul the whole thing (1500-2500). I just did the B3 disc and my tranny has never been better...mine was exhibiting much worse problems than yours but it took it about a 1 1/2 years when I noticed the delay in reverse.