I'm not sure about the licensing, I only have the 61c that I use it on. Rick from RSI is upfront about everything. I asked a lot of questions and didn't get surprised when the bill came. I don't think you need licensing for the other systems, but I think you need the licensing for extra phones(phone seats).
I'm not sure what you mean about spare parts. If it's spare parts in general, I've never had a problem finding parts.
One thing I forgot to add about the MerAssistant is once it's setup on a connected computer. You can remote into the computer a make telephone changes that way. Trying to get my IT guys to use dial up was like pulling teeth.