This is my personal view and others may have a different opinion on this. For majority of beginners and entry level turntable systems there maybe very little to be gained by using an external PPA compared to the built-in PPA.
The reason is because:
A. Most beginners will opt for a very cheap external PPA. It may sound different but it is debatable if SQ is actually improved.
B. Low to mid level turntable systems often don't have high enough resolution to take full advantage the more expensive PPA.
For a dirt cheap PPA that works OK try the Bozak Madisson M Phono Pre Amp or spend more for the Emotiva PH1. $87US

If you want to use an external PPA that is better I suggest Schitt Mani 2 for $150US
or the Rega Fono Mini A2D mk2 MM at around $200US.
or the iFi Zen Air Phono or iFi ZEN Phono 3 from around $130US to $250US.
or the Atom Phono which also has tonal controls $150USD.
If you want to UPGRADE your system I suggest you first spend the money elsewhere. However, when you have a turntable/cartridge combo worth around $1,000US or more and the amplifier and speakers are of a similar sound quality. Then it can be worth considering adding a nice PPA like this Cambridge Audio Duo MM MC for around $350US which also has a headphone jack.