Who ever suggested that those headunit are better should be slapped squarely across their face. Nakamichi is a Japanese brand and despite their newer products being made in China, their units are still far superior to those of rock mars etc.
Branded headunits with carplay or AA are kinda expensive. If you have anyone abroad e.g. eu or us, you can get units much cheaper there that support these features. If android auto isn't such a priority, you can find some really good featured pioneer headunits here for sale under 40k that will support front/rear/sub and features like sonic control, allignment etc
Usually you want to run all upgraded speakers through an external amp to fulfill the rms ratings. However, what most people do in a sq setup ( which usually are more front staged ) is run the front components and the sub from the 4 channel amp and the rear speakers from the headunit. A good headunit will provide 22-25 wrms to rear speakers so even if they are say about 50 rms, they do get sufficient power.
You might wanna look into the jbl stage 1 or gt5 components or pioneers entry level. Pioneers usually are a bit brighter ( some people like that ) while jbls and infinity are a tad bit neutral ( something you might wanna lean towards in a sq setup ).
I think soundworks in lahore is a good spot. Lazer sound used to be good, dunno what their situation is now. Hamza bilal is another chap to get in touch with.
The above equipment was bought from soundvalley in saddar and sent to my friend in lahore. So you can also do that if need be. You can call shehryar from soundvalley and give him my reference. Give him your budget and he'll suggest you the items