Get a well know brand,and find a good reputation shop to install it For branded head unit three hundred something and five hundred something (same brand) sometime might not sound different to yu .
To answer yr second question,dont turn too loud,and no if yu comfortable with it.Unless yu want to have good fun with yr music in yr car.

Shop around the price first for the Jpan brand yu wan (look for the feature yu need to use almost everyday)And dont
buy the feature/function yu usually lazy to use.And get the lower model which has the feature yu need.Remember to note down the model and price on each shop.
2)ask how do they do yr wiring (look like stock,or tide up nicely)
3)Yu need to buy front panel kits in order to install the new player unit on yr car dashboard,so ask about the price first
dont decide first come back home and think which shop could give yu the good price plus workmanship.
4)Dont bargin too long,cause yu are not going to buy on the spot,just ask second time for the best price people can offer yu.
So,Happy shopping