Once you've got fermentation temp, starters, and oxygen on your equipment list (all fairly cheap) splurge on a bigger kettle (15-20gal), and potentially a larger regulator for your turkey fryer if the old one isn't cutting it. Mine is 5psi which is fine for boiling 7gal of wort, but probably would have trouble with 15 gallons. How are you chilling your wort? If you don't have a great chiller, I would suggest buying or building one. If you're mashing/boiling in separate vessels, a pump can make your brew day easier too.