If you're located in the EU (I know, I know, that's a bit fluid of a situation for you right now), then I'd start looking at lead-free right from the start. As you stated, the exemptions are either expired or expiring soon, so you'll be needing RoHS in the future either way. Changing over the process can be expensive when you need to drain and refill a pot containing 1000-1500lbs of solder. That and it's just easier to have an entirely lead-free process thus reducing the risk of mixing in leaded solder or components like can happen if you're running both processes. Never had a problem soldering with lead solder and lead-free parts, unless the parts were heavily oxidized. Your flux choice will also have a huge effect on solder quality. Make sure whichever flux you use is compatible with the higher temperatures of lead-free solder and be prepared for a BIG learning curve to adapt to the process. Some PCB substrates can be horrible to get solder adhesion if you've had too much air exposure, but wave soldering with HASL boards isn't a problem like fine-pitch SMT can be.
It's a tough call between 100C and 305 for solders. The 305 is generally more expensive due to the silver content but has a wider process window when it comes to temperature, makes a difference if you have big transformers or insufficient hole size compared to the component leads. 100c is cheaper, has a tighter process window, but is much less aggressive at eating away other metals. They say it generates less dross but realistically you're going to make a LOT of dross with either type unless you have a nitrogen or other kind of inert environment like the Kirsten style oil bath. Personally, I went with 100C when I had to make the same choice. Cost was a big factor in that choice.
And that's a big kicker you'll need to factor in, dross. You can sell that dross to a recovery company, but you're probably going to use up more solder as dross loss than actual production. You also need to do regular solder samples and from time to time add supplements of trace minerals, we add a few ounces of Germanium daily at my full-time employer.