Car audio and speakers selection

Sirion/Storia M100
Hello, first post and all.
New to car audio but not soldering so...
I am looking for advice on what car audio to get. Low budget to start, but I supposed it can be upgraded.
There is currently only the standard VW T5 double din unit fitted, made from the finest Chineseium plastic, so the knobs break off easily, and an empty slot above for the mile log counter thing
The nature of the beast is that being a T5 Tucana coach built monster, any configuration is possible. Which is nice, as it currently has no speakers at all after ripping out the crappy tannoy install to announce the next stop. It appears to have space for the tweeters (small bird?) at the front. It's pretty big inside (much longer than standard) and don't know about a bass at the back.
My best guess is to put some sort of standard speakers overhead, mounted into the plywood A/C cover, or make some boxes. I can make this part of the conversion process.
Ideally I would be able to get speakers from of the shelf in GB, like Halfords.
Does the musical 'genre' make any difference to the hardware selection?
Could I get some advice as to what to spend my pennies on the start with?
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