Hi Nick, and welcome to SaleHoo, nice to have you onboard.
I would suggest that there is in fact a recipe book, perhaps not all the measurements are there, but if you're serious about building any business, including an online retail, I think that book is a well thought out business plan.
Biggest mistake I see people make in this industry is underestimating just how difficult and competitive this game is, then that quickly transfers to frustration and ultimately to giving up.
Make no mistake about it, online retail is a business and if you don't treat it as such then making a success out of it can be extremely hard!
I can see that from what you have written already that you have some firm ideas on what's suits you and the direction in which you want to proceed, and that's a great start to any business plan, so build on that till you know where you are now and where you want to be in 5 years time, and have a road map as to how to get there.
I would suggest, as I do with everyone, kick off with second hand. eBay is a bargain hunters paradise, feed the beast bargains, and it will eat them quicker than you can list them.
Areas such as Garage Sales, Weekend Markets, Thrift Shops, even Local Classifieds can all be sensational sources for inexpensive, but quality and popular products. Even better if your not shy and can talk a good bargain!
Yes, it does take more work than drop shipping, no doubt about that, but it does have several strong advantages over drop shipping in my opinion. Variety is the big one, you never know what you'll find, and keeping your items fresh can mean that you can keep your customers coming back again and again.
Price, yes it is money upfront, however the price you can pay can leave you with good margin to sell on sites such as eBay, but you do need to know the value of items and be able to talk a good deal.
A unique position in the market, everyman and his dog are selling iPhones, but by sourcing second hand you have the opportunity to get your hands on products that aren't saturated on eBay.
Of course it is also a great, and relatively inexpensive way to do market research with products.
You sound like you have a good head on your shoulders, and being a mathmatician you will have an understanding of 1 + 1 = 2, or in business terms, good planning equals a good chance of success : )