Hi there!
You can use any tape but you might want to use a "High Bias" tape and keep the length kinda short (under 60 minutes). Tapes that hold more music like 90 or 120 minutes tend to be a bit thinner and stretch over time. Cleaning the play and record heads with some denatured alcohol (I got mine at Home Depot.) and a firm Q-tip is probably a good idea too. Don't get any on rubber parts though. All this should help keep the noise to a minimum and maintain better sound quality.
A home stereo tape deck will work to mix down on. You may also want to clean the heads on that deck as well. If you have some kind of noise reduction system like dolby, that can also be used to clean up the sound as well. Technically, the heads on both the 414 and home stereo deck should probably be demagnetized, but that may not be necessary depending on your needs. I hope this helps.