Move mail from older Mac to new one

Just got my new iMac 27" and I'm debating on how to sort through all my email accounts and purgejunk, keep stuff I want. Should I do some bulk deleting of junk mail on my older 2007 iMac before attempting to use the transfer assistant or should I just bite the bullet and send everything over? The only stuff I'll want to keep is probably family emails with pictures embedded in emails. If there were a way to select email parameters like senders and recipients, that would be helpful.

I have comcast/Xfinity email accounts, and three accounts. Do I just copy all the setup info to the New I iMac and it'll magically appear? And then purge and reset the old Mac? My intention is to sell the old iMac, so I have to keep that in mind. I don't want any ghost emails or other data still hanging on the old macs HDD.

On my old iMacs 500GB drive, I have about 145 GB free. The new one has a 256 Gig SSD. I figure I can add external storage as needed.

You have a housekeeping task ahead of you whether you delete before or after your transfer; can't see any difference on when you do it. Migration Assistant will replicate all your data on the new machine.

select email parameters like senders and recipients

You can choose what you want to save by using the search box at the top right of your email accounts. Highlight your email address in the left hand menu > put a name/firm/subject etc in the search bar, press return, and all emails featuring that person will come up > show as a list > highlight all and save to a new folder (File > Folder).

Repeat for other recipients etc.

You can then delete all that isn't in the new folders.

I use Thunderbird but the facility is the same in other Mail apps.