The 124 MB does NOT convert well. I read where one of the auto a/c specialist shops highly recommends NOT converting them. They had converted their own and it was a disaster, they ended up reversing the conversion.
R12 is now at about $29 per can and coming down. You can recharge for the same money as a conversion or less and have proper cooling. Converting these cars to 134 can never achieve the cooling capacity of 12 and you will have MUCH MORE invested than if you bought 4 cans of R12. You can add a parallel flow condensor and other expensive things and still not achieve what you are looking for. The a/c specialist mentioned above did all these expensive things and gave up.
Additionally, the added pressure and less effective lubrication will lead to earlier failure of components.
I have reverse converted a system to R12 that was previously converted to 134. You have to thoroughly flush everything, replace the filter drier, pour out as much oil as you can and put in the correct amount of mineral oil.
I would highly recommend staying with R12 if you want to stay cool in this car. There are many cars that convert okay, but you're not driving one of them.
Best of luck,