I’ve stumbled across a good deal on a Rega Planar 3 fitted with a Grace G-747 that looks to be an excellent choice for my second / MM table. It has an old ADC on it that almost certainly needs a new stylus and I was never very fond of the ADC sound to begin with. I’ve had good luck with B&O, Ortofon, Grace, Empire and Shure, but everything except Ortofon is ancient history now.
Replacement styli for Grace F9 are available from SoundSmith and all you need is a Grace cartridge body if you don’t have one.
I prefer original Grace styli and not even F9 cartridge body (I prefer F14 and LEVEL II), but Grace F9 is one of those inexpensive vintage MM cartridges anyone can buy today without a fear of unobtainable styli. SoundSmith styli always an option (when they are available).
I still have NOS (factory sealed) original Grace styli for F9, F14 and LEVEL II.
However, your $300 budget is too low for a good MM cartridge or even for a replacement stylus alone, you have to double your budget and then you can find really good MM cartridges with advanced styli.