Mains-powered project safety

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I am building a general-purpose inverter (60Hz AC --> 40-140 kHz AC) for powering flyback transformers, induction heaters, etc. The project link I'm referencing can be found here: http://uzzors2k.4hv.org/index.php?page=multiinverter I am aware of the hazards present from the mains-level voltage; however, what I am more concerned about is the fact that schematic shows the a direct connection to unisolated mains.

If it is possible to do so safely, I would like to use this circuit without an isolation transformer. However, that brings up several question:

1. The author built his inverter within a salvaged ATX power supply metal enclosure. Can I ground the metal case, or would this cause problems due to the fact that the circuitry runs unisolated?

2. If I did ground the metal case, I would certainly need to ensure that the MOSFET heatsinks did not make contact with the case---correct? Is there a practical way to isolate the heatsinks from the case?

3. I realize it would be exceedingly dangerous to leave the case ungrounded, with the unioslated circuit. If this were necessary (due to 1 and 2), I suppose enclosing the ATX case with plastic would provide enough safety?

Also keep in mind that I would be the only person using this inverter--it will not be sold/installed/used in an environment with anyone else, just to clear up potential liability concerns.

Thanks!