155's were made in 1967 and 1968. The hydraulic implement lift was an option but it was also sold as a kit that could be retrofitted to many of the earlier tractors, including Colts.
The year of the tractor you are selling is pretty immaterial to any buyer. Condition matters most and to some collectors, the brake issue might come into play. 155's are the most abundant of the 100 Series GT's and usually bring around the $300.00 mark, on average.