Newbie questions

[Mannysrt8]

Need some advice from the audio experts.

So i am in the process of replacing the base unit,i do not have the Bose just the basic piece of crap Nissan put in there,very surprised how a 35k car can have such a crap stereo.

So i am looking at few different Double DIN units with Navigation,however I might as well replace the speakers in the door and up on the dash.

I like to be frugal so I will probably look for a used double din and speakers...can someone tell me what direction to take on which Double Din is good but will not break the bank.Also when shopping on ebay what size speakers should i be looking for to replace the door and the dash speakers.

Once installed is it easy at a later point to install a back up camera?Or should i be doing it at the same time

Thanks in advance

Manny