While I think it's a grand idea selling PCs pre-installed with an Amiga Operating System, unfortunately it's illegal to sell copies without a licence, which is just about impossible to get now. Selling a PC with Amithlon preinstalled wouldn't be any less illegal than selling a PC with copy of Amiga Forever 2009, for example. It doesn't make a lot of sense, but it's just the way it is unfortunately.
If you'd like a completely legal way of reselling PCs with an Amiga OS on them, perhaps you could look into Aros! It's an Open Source reimplementation of Amiga OS and is source compatible. There is quite a bit of software available for it now, and it has a built-in UAE to run older Amiga software (you only need to provide your own Kickstart file).
The site you mentioned sells flashable EPROMs as backups for real Amigas with failing ROM chips. The service they provide to backup your Kickstart to the EPROM for no extra charge isn't intended as a source for pirate Kickstart ROMs.
No one wants you to give up on your idea, you may just have to rework it a little. If you contact Cloanto, there's a chance they may offer a reseller deal for their Amiga Forever package, which you should be able to sell with your PCs preinstalled, as long as the original CD is provided with the PC.
