If I could redo my first grow, this is what I would personally buy:
Tent height matters - having that extra couple inches to crank up the lights at the end of a successful grow just saves you headache. A square footprint is also just easier to work with illuminating.
Buy once, cry once with the AC Infinity fans. They're absolutely worth it, and the bigger the better because you can run them at lower speed to keep your grow quiet! Plus, a 6" kit can be used if you decide you want a huge tent later, or if you need a fan for exhausting your attic or a basement - they're the real deal.
Spider Farmer Bar Lights - I bought a G8600 out of my own pocket, and it's the best thing I own. Spider Farmer sent me a freebie SF-1000 board I'm trying out, and while I like it, the bar-style lights are substantially more powerful beasts.
If you have the budget for an SE-5000, that would be my dream light for a 3x3x6 grow, you won't need the full wattage, but the fact it covers the tent from edge-to-edge in lights means in terms of maxing your yield within a small space, it's hard to beat. If you're only going to grow autoflowers, or use the tent for vegging exclusively, a G3000 would be more than enough, an SE-5000 is serious overkill.