How important is the data you're usually recovering and how much profit do you stand to lose if a recovery fails? How much is a stick of freaking RAM? Just max it out, for jabu's sake. Yes, you won't need much for most recoveries. But data recovery is EXTREMELY important as the data is IRREPLACEABLE (that's why they're PAYING you to recover the data!). I use high end i7 systems with 64GB of RAM. I wouldn't specifically choose a motherboard that supports 128GB of RAM just to get up to 128GB (as the motherboards/processors are quite a bit more expensive, and you don't NEED those faster processors for data recovery), but 64GB is more than enough. You do NOT want to run out of RAM during a big recovery job and be caught with your pants down. Sometimes you only get ONE SHOT at recovering the data. You don't want to lose your clients important information (and yourself good profit) because you decided to cheap out on your data recovery station.
Now I understand that not everyone has a budget for this. If you're just starting out and just scraping by, you can get away with 16GB if absolutely necessary. But I'd get that up to 32GB ASAP, and invest 100% of your recovery profits into getting a board that can support 64GB of RAM. A technician is only as good as his tools. If you don't reinvest back into your business, you'll lose money in the long run.