Bosch Fridge Freezer Excel Logix?

Hi, the freezer compartment of my fridge freezer has 4 drawers but only the bottom drawer's contents are freezing...the rest have led to food defrosting and being thrown away. a local engineer attended recently due to the freezer temperature having risen from -20C to 0C. He took off the interior rear panel of the freezer and found a significant build-up of ice which he cleared by using a hot steamer device. This caused the freezer temperature to rise to +7C but then eventually after approx 12 hrs it returned to my chosen pre-set temperture of -20C.....but only the top shelf is freezing any food from fresh. The engineer has already charged me a call out and labour fee of £78 including VAT but says that the likely cause is a wiring problem which cannot be repaired as the wiring is encased within the foamed casing, he informs me...so has said that the appliance is effectively useless. That said, the fridge seems to be operating ok at my chosen temperature of +2C, but he says that this too is likely to fail in the near future.

Am I now left with no choice but to buy a new one, or is this problem repairable please? The appliance is approx 8 years old.

Many thanks in advance.

Andy