Honestly I wouldnt hire a shop to do it. They are always wayyy over priced and we can walk you through anything you need to know.
Im trying to save you money and skip the headunit all together. Its a unconventional way to connect but i think its a Great way to stay stealth about it all. The iphone is tricky but you can always run a cheap android to it.
A good speaker is only as good as the install. Seal and deaden your doors correctly. Have your sub in a box thats correct for it. So yes, get your speakers/ sub installed first. Then onto amp settings, crossovers, time alignment and then lastly frequency response.