Another vote to stick with the r12.
$600 is too much, and conversion is not related to troubleshooting, so this is suspicious behavior on their part.
$600 dollars as a starting point to save $120 on refrigerant? Hmmm... Even with recharges every 3 years, it would take a long time to pay that back...
Hey, you could even get lucky and find that you simply have a low charge, which is normal over time.
1986 300E 5-Speed 240k mi.