If you are thinking of running any kind of business on the internet you have to realise what makes you unique, and play on that. Otherwise you will just drown in the sea of identical websites.
Based on that I would say forget the non ABDL side of things altogether. Theres already hundreds of places that already cater to this and with all due respect far better than you can as one small business.
If you were a medical supplier already, I would maybe suggest this route but since your not I think you should steer clear.
There are several things you can do to compete with existing ABDL shops. If you can knock up items out of wood like cribs, bespoke or otherwise, thats probably the main thing you should be using to draw in the customers. The diapers and other consumables are what bring in the day to day profit.
A wide range of products will help set you apart from existing ABDL webstores but the crucial thing is reliability and customer service. That means you need to convince the customer somehow that you arent going to disappear overnight with their money, and that their packages are going to arrive discretely and promptly every single time. One way to acheive that would be to start selling initially to the DD community, using your presence on here as a place for feedbacka and a guarantee that you are standing by your products. But this idea is not sysnonymous with rapid growth, and of course you would need to compensate DD for using their services.
I do beleive there is still a niche for this type of service. There have been a few attempts in the past which have only failed due to typical ABDL personality shortfallss (insanity, instability and retardation).