Classic Pioneer table -upgrade cart or replace?

Hi all, I am an analog novice and have a 1989 Pioneer 518L direct drive table with one foot missing and one spring from inside another one of the feet missing. It still plays properly and has adjustable speed with strobe so I assume it plays at proper speed. However, it doesn't sound much better than my Sony cd player in my system. I have about $500 to spend on used equipment. SHould I just upgrade the (very) old audio technica cart, or buy a used table with tonearm and cart included? Also what table/arm/cart would folks suggest in my price range. Thanks for the help. Dave
Bought a Mint direct-drive Pioneer PL-660 TT on Ebay for a song and replaced the the stock conical P-mount AT needle w/ a much better Audio Technica AT 311EP cart.($40 J&R Music World) for a total cost of $85. Sounds pretty good to me. The TT is really easy to use. My other TT, an entry-level Project, has a Sumiko cart. (Blackbird, I believe)which sounds pretty decent as well. These run around $60 I believe. I remembered I liked the AT 440ML cart. as well. (Don't know if these are still made?) The Denon DL103 High Output moving coil was quite nice as well. How good your phono pre-amp is, has a great deal how good your TT/Cart. will sound. Hope this helps. Bill