I I was to start piecing together a system, where should I start?

I have an 01 convertible. I want to eventually replace everything. Non HK if that's relevant, although I'm not to interested in BSW. Unless a stupid good deal comes around.

I'm looking for a head unit first, and a question arises there. Are all head units the same size? With a bracket I've yet to locate would they mostly fit into it?

Thanks ahead.