An ecommerce site or store? I'm assuming it's an online store.
Sidetracked from market research....
I think it depends if you house your own inventory or not. That plays a big factor in choosing your stores products. I was looking for experience when I started my store and not as much for a big market share(or money), so I just looked fo teh best supplier I could find. I found a supplier wtih $1.25 billion in sales, their CEO was ranked 20th in the US, adn were voted the best supplier in the US(not to brag

). They had alot of audio electronics, so I went with audio electronics, even though it's a saturated market. I did this because I coudl focus on the other aspects of business(remember, I did it for the learning experience) besides dealing with a crummy supplier, which I think alot are.
For market research... I would go to forums that relate to your possible choices and ask around, get all the feedback you can and also search for every possible competitor (I researched my closest compitors,and the closest I found were
www.audiocubes.com and
www.beachaudio.com (which uses my dropshiper

)) and see how you can beat them or if you could do better than them. Ecommerce market research is harder to conduct because the internet is so big and it's hard to cover the traditional market research aspects. Make sure you believe and enjoy the subject for your idea or you won't have the motivation to finish. ;-)
Another factor is how big you want your store to grow. You said you have alot of resources and if you want ecommerce to be your full time job, than definately look for an area that has a weakness, as opposed to trying to steal market share and force your way into the market market, ex. book stores. an example for smaller idea is selling soemthing people have fetishes or collect for, like used socks, pez dispensers, cookie jars.
So sum everything up, I personally think ecommerce research is a different type of market research(maybe it's not different?) and should analyze the amount and quality of competitors, suppliers and whether you hold the inventory, the price range fo your products, and how big of an audience your targeting. I hope I helped. : )