The Oyster replacement stylus is $25. bonded spherical tip.
So you can obviously do a lot better, though as always the laws of
diminishing returns take hold with a vengeance if you spend too much.
I'd say the arm is compatible with just about anything mid-priced.
Go for a high quality nude tip as a major upgrade over the Oyster.
In the states the Audio Technica's $50 AT120E/T (nude eliptical) and
$85 AT440ML (nude fine line) are outstanding value for the quality of tip.
The Denon DL110 and DL160 also have high quality nude tips.