Any other advice for importing all this before I do it?
Do them in small-ish batches...
PT used to have a big problem when importing vast amounts of data, which resulted in the import speed slowing to a crawl. They made very good strides to reducing this effect a few months ago.
However, I would still split the imports up into batches, import each one, stop the import, let PT do the database maintenance and then start a new one. I have found this ensures consistently high import speeds and also that if you do have a hand history file that causes an issue, you can quickly identify it and deal with any problems.
Furthermore, I don't know why you want to import every single hand you have ever played, but I would make sure you put them in a separate new database. I can't imagine you want to mix your last months results with really old results, or use 4 year old data in your HUD. And if you were to mix the two, the speed of getting your reports up from a massive database, when all you want to do is look at day to day results will be really slow.