Two things to remember:
R12 is expensive so if you stay with it, it'd be worth your time to get a leak detection dye just in case to find out if there's a leak so you can fix it after it leaks out.
Another is a small leak with R12 is a huge leak with R134a. The molecules of R134a are quite a bit smaller than R12 so they leak real fast, but thankfully it's inexpensive and doesn't harm the environment. Also, getting dye in it if you convert would be a good idea to see if there's a leak to fix.
One more thing:
The best lubricant for the lines is Ester, I've heard that PAG can discolor your skin if you get it on you, especially if the AC system decides to blow up when you're tinkering with it.
Good luck!
------------------
'84 Mustang 5.0 LX
My car