There is a discussion (and comparison with a Technics SL-D2) about the PL-518 on AK here
http://www.audiokarma.org/forums/showthread.php?t=466901 , as well as other DIY refurbishment threads here
http://www.audiokarma.org/forums/showthread.php?t=473889 and here
http://www.audiokarma.org/forums/showthread.php?t=333152 . The main problem with them seems to be the feet, on which the rubber falls apart, but the DIY articles show how to fix them – it is an easy fix. There are probably other articles on them if you do a search.
It was a very popular direct drive turntable from right in the middle of Pioneer’s range in the late-1970’s, with a heavy wooden base. It cost about three quarters of the price of a Technics SL-1200MkII, and unsurprisingly wasn’t quite as good for performance or features (no quartz lock or tonearm height adjustment, for example), but did have the advantage for home users of being semi-automatic (auto return at end of record). However, it’s a very solid and reliable model that was a very good performer, and as millions were sold, spare parts should be available – you may be able to find a broken one to use as parts. Pioneer back then had very good turntables that have stood the test of time. Even if you have a problem with it, as so many were sold, you are almost guaranteed that the answer to fix it will be on forums like this.
The cartridge is Pioneer’s PC-135
http://www.vinylengine.com/library/pioneer/pc-135.shtml , and should be a reasonable match with the tonearm – the correct tracking weight to use is in the 2-2.5gram range. The replacement stylus is widely available from places like LPGear
http://www.lpgear.com/product/PIR026.html or from eBay sellers such as Carole’s Needles
http://www.ebay.com/itm/STEREO-NEED...rd_Player_Turntable_Parts&hash=item2eba09b3e6 – just about anywhere really, as they were very popular.
As Balifly said, an upgrade to a better-sounding cartridge could be made easily. One that has a recommended tracking weight in the range 1.75-2.5g will be a good match with the tonearm.
