Vintage DD turntables. Are we living dangerously?

I have the GT2000 and am quite happy with it, though I have not done direct comparisons with other DD turntables.

The difference between the GT2000 & GT2000X is that the latter has a bigger motor shaft and plinth. The have the same platter, motor, and controller. The 18kg gunmetal platter was optional for both.

Some people like the arm on the GT2000 but many also change it for something else. Yamaha sold two other arms and SAEC made a special version of 407, the 407/GT that can directly replace the existing arm. There are also commercial arm boards available that allow the use of other tonearms. I have no experience with or knowledge of fractures in the tonearm.

No. I've compared it to a Garrard 401 with the same cartridge (SPU Gold) but with different tonearms and preferred the GT2000.

I haven't seen either the GT2000X or the YGT-1 platter in real life. They're both pretty rare but do occasionally pop up on the net.

Regarding the inertia of the platter & servo mechanism, Yamaha GT2000's original platter is ~5.5kg, but Yamaha offered as options a vacuum platter mat (~2.2kg) and a gunmetal platter (~18kg). So if you used the vacuum mat with the gunmetal platter, you'd be at roughly four times the original platter's weight. The shape of the gunmetal platter is the same as the original aluminum platter's shape, so presumably the inertia to weight ratio would be about the same too.

So at least in this case, the manufacturer did not consider a very wide range of weights to be a problem.

GT2000/GT2000L/GT2000X seem to sell as soon as they are posted on Hifido. They probably have a waiting list. And I think the P3 there now was reserved pretty quickly after it was posted.

Lewm: That reproduction brace is indeed attractive, but costs 700,000 yen or $6.5k USD... 

Henry & Lewm: The going price of TT101 indeed is on the low side.  I recall a couple of years back seeing someone sell a TT101 with full maintenance done by the seller for >$1000 on Yahoo JP for just the motor, but I failed to bid then and haven't seen them come back again. 

The only differences I've been able to find between GT2000 & GT2000X are the better bearing, plinth, and tonearm in the latter. This has also been confirmed by someone who has worked on both. The platter and motor as well as the motor controller appear to be the same. GT2000X usually sells for multiples of GT2000. 

That gunmetal platter weighs 18kg and usually goes for more than the turntable itself. Here's one for sale:
http://page6.auctions.yahoo.co.jp/jp/auction/f174978328

The metal frame that lewm mentioned was an optional accessory that appears very rarely on the used market. Here is a current replica of it, though in gunmetal rather than cast iron:
http://www.ne.jp/asahi/y31/pro/y31ab1/phot.htm

All the other original options are briefly explained here:
http://audio-heritage.jp/YAMAHA/player/gt-2000.html

ddriveman: I tried the mu metal shield and ERS paper on my Yamaha GT2000 between the original platter and the copper platter mat. I found the ERS paper to give more refined highs, but that could have been due to its damping effect as a slightly soft layer between two metal layers. The mu metal didn't seem to have a beneficial effect and may have had a slight negative one. Anyone else with experiences?

The only differences between the GT2000 and GT2000X are that the latter had brown veneering, bigger plinth, bigger motor shaft, and better tonearm (sold as an option on GT2000 as YSA-2). The bigger plinth & bigger shaft cannot be installed later on the GT2000.

The only differences between GT2000 and GT2000L are that the latter had the automatic arm lifter at the end of the record (optional on GT2000 as YAL-1) and brown veneering (black on GT2000).

Like Theophile said, the 18 kg platter worked with all three models: GT2000, GT2000L, and GT2000X, though undoubtedly the bigger shaft on GT2000X had benefits.