I think most 1200 users agree that the best upgrade for the Technics is the Sumiko headshell. At $40, it's also the most inexpensive.
http://www.needledoctor.com/Sumiko-HS12-Headshell?sc=2&category=432
I believe the fluid damper only makes a big difference when using finnicky cartridges that cost far more than the SL-1200 itself, like low-compliance, low-output moving coils. So if, as you say, you plan to use a high-end cartridge, then absolutely, go for it. I find Denon MC cartridges pair very well with the SL-1200. The most popular pairing is with the DL-103 and DL-103R (which perform beyond their modest price tags) but the $950 DL-S1 is one of the greatest steals in all of audio. Just some suggestions.
Try the Sumiko first. I consider it essential so regardless of whether you do the fluid damper or not, it won't be a wasted purchase.
http://www.needledoctor.com/Sumiko-HS12-Headshell?sc=2&category=432
I believe the fluid damper only makes a big difference when using finnicky cartridges that cost far more than the SL-1200 itself, like low-compliance, low-output moving coils. So if, as you say, you plan to use a high-end cartridge, then absolutely, go for it. I find Denon MC cartridges pair very well with the SL-1200. The most popular pairing is with the DL-103 and DL-103R (which perform beyond their modest price tags) but the $950 DL-S1 is one of the greatest steals in all of audio. Just some suggestions.
Try the Sumiko first. I consider it essential so regardless of whether you do the fluid damper or not, it won't be a wasted purchase.