Twl is right. If you have the original arm it won't be a good match for a Shelter or other low compliance cartridges. That arm was designed for lighter weight, higher compliance cartridges (as were most arms of that era).
There may be "some" $8K rigs that will outplay a modded TD125. :-) There's more to a good rig than weight. Nevertheless you should be able to get very satisfying music from a well maintained Thorens. You might visit User510's Thorens site, www.theanalogdept.com. Lot's of good info and Steve has lots of cartridge experience on various Thorens arms.
There may be "some" $8K rigs that will outplay a modded TD125. :-) There's more to a good rig than weight. Nevertheless you should be able to get very satisfying music from a well maintained Thorens. You might visit User510's Thorens site, www.theanalogdept.com. Lot's of good info and Steve has lots of cartridge experience on various Thorens arms.