If I was in your position, I would take it to an MB mechanic and have it recharged, inlcuding uv dye. Next time it leaks out, you can find out where and fix the problem (or have it fixed). It's not worth trying to recharge yourself as without gauges you'll risk overcharging and thus damage.
1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick