Understand How & Why a Dev Kit Should Be Your Design’s Starting Point article tells that many electronics designs start with a development kit. That’s a very good point to start. The processor vendors usually offer them at very reasonable prices, or free in many cases.
I think that another good starting point are many open source hardware designs that are available. Examples of such I have mentioned on my blog earlier are various Arduno versions and several ARM dev boards. Especially on low volume products starting from board you can get easily ready made is a good option, and because design is open you can later arrange your own board manufacturing and make your own board version you see needs for them.
I just started a new blog postings category for dev boards