Where to start....
Don't worry about what one person thinks of an instalation centre, people are likely to tell you all about the bad experiences they had, and why you shouldn't go some where. Ignore this. If you are really serious about having something properly installed, go into a few installation centres, and talk to the installers. Ask them if you can see some of their work, or what their working on at the moment. A good installer will be happy to take the time out to talk to you (or otherwise arrange a time when they are not so busy) and to show around to some of the work they are doing. Also remember, you can request that a certain installer do the work for you. Some where might have 3 installers, 2 of which basically couldn't give a ****, about you, or your car, while the 3rd guy, might live and breathe car audio, and wouldn't think about doing anything less than a perfect job. Something to think about....
Secondly, all one brand?? Your obviously new to the audio scene, so i'll try and explain it in easy to understand terms, as to why we don't generally reccomend this...
Lets say you and i both make car audio components. Now my brand, i make some of the best head units on the market, as far as bang for your buck is concerned. My speakers are rated as being some of the best out there aswell. However when it comes to my subs, and amps, i have a very limited range, and do not offer the type of performance levels found by some other manafactuers.
Your speakers and Head units on the other hand, aren't the greatest,as it's not your main focus. But your subs and amps are amongst some of the best that money can buy. Now would it not make sense to grab the best components, from both of our brands, and then put them together? Of course it would.You need to look beyond the brand/ appearance, when buying something. Let you ears do the deciding. At the end of the day, it isn't what you see in the car, it's what you hear.
Other than that, pretty much just take your time, and don't rush things. Listen to as many different componets as you can. Also remember, when being shown something on a demo board, ask questions like which amp is running these speakers, or this sub now? A lot of the time, you will hear something in a store, but when they offer you the package deal, the amp is not the one you heard on the board.Or ask that they play the music through some of the head units that you are considering, or are within your price range. A lot of store's will go straight to the highest end head unit, when demonstrating something.