So my dad is retiring after 35 years in the army and DOD civil service to a really nice place that mom and him bought cheap when the market in Phoenix tanked. He and mom intend to remain residents of Idaho where they will spend summers and winters will be spend at "Casa El de Fsjdw2 Sr" (hand done mosaic on bottom of pool, jacuzzi, sauna, 5 bedrooms, gated community, ect ect) . Dad's firearms collection consists a .38 antique from wwII(uncle's bring back), and a .30-06 remington 700 both at the summer home in idaho. I will be visiting in winter and am curious about  gifting dad a stripped lower and parts kit so dad can build a replica ar15 (m16a1 replica with no switch). Will I legally have to transfer the lower through a dealer? If so who is a good dealer(cheap, and stocks some .223 for sighting in reasons) in the Peoria area?

Welcome to AZ.

It is legal to gift a firearm, no dealer required.  

Unless the OPs profile is out of date, he is in Alaska and his dad is splitting his time between Idaho and Arizona. Interstate transfers must take place through an FFL, even if they involve family members. Now, if the OP were a resident (not just visiting) of either Idaho or Arizona he would be able to transfer the firearm while his father was resident in the same state. ATF FAQ for guidance.

If the OP were to ask his dad to keep one of his guns at his dad's AZ home so he can use it when he visits I can't see any issue. His dad could even take it out and do a little shooting to make sure the gun is sighted in and functions OK.

I would suggest just taking him to a shop and picking up a lower for him and with him!
Quick Guns Mesa
Scottsdale Gun Club
Shooters World Peoria (2 locations).