If you insist on converting to Freon 134 from Freon 12, that's all well and good. Although I don't convert them any more. I have converted several, some of which I have since converted BACK to Freon 12.
To begin with, the price of Freon 12 is actually falling due to the fact that the cars that require it are at least 10 years old, so they are hitting the wrecking yards every day in large numbers.
Freon 12 can be purchased for $29 per can or less with a 609 certification, which can be obtained by taking an online test for $15.
Steve Brotherton says it best when he states that R12 is the least expensive major component of your a/c system.
Many people are penny wise and pound foolish. They will spend scads of money to convert a system so they can use $5 per can Freon 134, instead of just repairing the leak, which must be repaired in any case and staying with $29 per can Freon 12.
All that said, regardless of which refrigerant you choose, you will need much more equipment than a set of gauges. You will also need a vacuum pump, a flush gun with solvent, and other various tools.
Don't even THINK about charging a system without evacuating first.
One more thing. I also bought a Haynes A/C manual. It is quite aged, and there is much that has been learned about Freon 134, it's lubricant compatibility and other things since it was written. Additionally there is some information in that book that is just plain WRONG. As an example, they are proponents of turning the refrigerant supply cylinder upside down and charging the system with liquid. This is a risky method that can ruin your compressor.
I would recommend that you throw it away or tear out the pages and use them to line the parakeets cage, and buy the manual available at www.aircondition.com or www.ackits.com.
You will also find the discussion forums at both of these sites to be extremely helpful.
Best of luck,