Answer: I am a purist at heart, and I like to keep things original when possible, but sometimes other factors such as economics and reliability must be taken into account. Unless your car is a very high quality “survivor” or has been faithfully restored to original, I would have to agree with your mechanic.
The gauge(s) in your car can be repaired, or even restored, but the labor to remove them, repair them (they will have to be sent out for this), and replace them will be expensive. Based on the last ones that I did, I would say in the range of $1,000.00 for all of the gauges. It really does not pay to tend only to the temperature gauge. The original gauges in your car were not particularly accurate even when new, so even after repair they will not be as accurate as a high quality set of gauges of the type that your mechanic is recommending. It is for this reason that performance enthusiasts installed these aftermarket gauge clusters on their cars when they were new, even though they came with factory gauges in the dash. Although not original, I use the term “period correct” when I see these clusters hanging under the dash of a collector car.
Make sure that your mechanic uses gauges from a manufacturer known for their quality, and make sure that he hooks up the light kit so that you will be able to see them at night. This is often overlooked. I also recommend that he use an electrical oil pressure gauge rather than a mechanical one which requires a pressurized oil line to be run into the cabin of your car. Should the oil line pop off of the back of the gauge - and I’ve seen this happen - you will have hot oil all over the interior of your car.