Well, besides a few small parts that mostly don’t affect sound quality, the main difference between the top model Series Five and the budget Classic is the bridged arm yoke of the former. Some of the parts standard on the Series Five arm are offered as upgrade options (stainless steel counterweight, alloy armrest, headshell shims) on the Classic, but when you add in the cost of them, you may as well have just paid the extra (the Series Five is 625 British Pounds, the Classic 455) and gone for the S5 straightaway.
The bridged arm yoke is NOT available as an upgrade for the Classic, and if you decide you want the micrometer (invaluable for fine adjustment of VTA/SRA), it also is not offered on the Classic. It is a 220 Pound option on the Series Five, only. The prices above, incidentally, include internal wiring of litz silver.