I’d recommend importing the data, then re-running the import once that’s complete with the last week’s worth of data. The first import will be what is most time consuming. It would probably take 1-2 days to completely run and then review the import process for the first bulk of data, depending on a number of factors.
Based on the little bit you’ve said it should, it really depends on if you have anything extremely custom that you’d need to retain, if you’re need to implement it into Discourse or pay for someone else to then you should look into that before moving over to be sure you’re confident it’ll be possible.
Be sure to check these two links:
That should give you a rough overview. If you have further questions or needs you should search around on here to see if it’s possible to do everything you want or ask here and I’m sure someone will be able to answer it.
I personally am migrating one of my larger Xenforo boards to Discourse, to ensure a smooth migration I just decided to get their paid hosting, better to have hands-on support by the team that developed Discourse than winging it on your own, but that was more for the migration than anything else. However, having peace of mind for the next year of only having to worry about running and growing the community is a great relief and time saver. That also would solve your issue of if you get run over by a bus someone could still manage the system for you.
Convincing members seemed to be pretty easy for our board, the only challenge was they aren’t used to the layout so that aspect was the toughest sale, the improved features and performance won over the vast majority of the community. It is certainly a conversation you will want to have with your community though.
You’re in luck too, the phpBB3 importer seems to be their most well built one. Here is a link to the howto article on it: