vera6.4 - First post, Welcome to DealershipForum.
I'll defer to anyone who has more knowledge, but if it were me I'd try to encourage the customer to wait four months until the Remote Start Kit is available.
In my experience it seems like trying to piecemeal an accessory package together almost always turns into a headache. Even if you obtain all of the parts there's no assurance that they'll all work together without flashing the ECU or making some other adjustment. Additionally, it will likely be much more expensive to go the piecemeal route and your tech will be on his own if he needs any troubleshooting support.