I think there are better approaches that won't cost any additional money.

For starters, the venturi vacuum pump won't do the job. You need to pull a deep enough vacuum that water boils inside the system - difficult or impossible to do with a venturi. And you need a set of guages to insure the system doesn't leak down and also to verify proper operation when you get it running again.

Autozone is pretty good about loaning vacuum pumps, and possibly guages. You basically buy the pump ($200), they refund your purchase when you return. I'm not positive they loan guages, but I've read (here) that Harbor Freight has them for ~$39. Seems worth the investment. Might be able to find guages at a rent-all type of place.

Because the system has been sitting open for several days, you need to replace the receiver/dryer. It's an easy task, just be sure to get all the o-rings for the inlet/outlet and switches on the darn thing.

You'll need to add oil for certain. Replacing the evap and dryer will require - I'm guessing here - four ounces. Best bet is to purchase a bottle of PAG46. On a 124 it is very simple to open the low side line just in front of the forward firewall - there is a threaded connection there. The oil can just be poured in. You'll of course need to replace that o-ring as well. Don't forget to lube the o-rings. Refrigerant oil is good, Nylog is better (if you can find it).

Vacuuming does not remove any oil. You just need to make up for what is lost when the old evap and dryer are changed.

I agree that $40/lb for R-134a is too steep. Even with the price increases this year, single cans are going for $11 in my part of the woods.

Your system likely calls for 1.0 kilogram of refrigerant. Conveniently, this is almost exactly 3 12oz cans. You'll be undercharged an ounce or two, but that's better than overcharged.

With the system under vacuum, it will draw in almost the entire first can of refrigerant. Before starting the engine, add the first can as liquid to the high side port. When the flow of refrigerant slows, close the manifold valves, start the engine, switch on the a/c, and continue adding through the low side port (as vapor, of course). It doesn't take all that much refrigerant to trigger the low pressure switch.

And don't forget to purge the air out of the manifold hoses before adding refrigerant!

I'm going to be offline for about 10 days. Best of luck. I expect cold air when I return!

- JimY