I have been around electronic test equipment for 35 years. There are many good digital meters for very little money these days. Because of the car you have, I would strongly recommend a little meter from Sears that has a Duty Cycle function that you may very well need at one point for Lambda adjustment.
This meter also has provisions for a thermocouple which is extremely handy for checking evaporator temperature. It also has a frequency measurement. Freq meters used to be a very specialized piece of equipment, now it's in a $30 meter!
I bought mine at the store a few years ago, but it can only be had from the online Sears tool store now.
The Craftsman number is: 82139
The Sears Item number is: 03482139000
I have been completely amazed by this meter and its quality and functionality for the money. I have a number of different pieces of test equipment, but I find myself reaching for this almost exclusively. I highly recommend it.
BTW, it is $29.95 from the Sears online store.