Okay, well I guess it would be helpful to know a little more about your setup. What are you brewing with now? What is your capacity? Can do you do full boils?
When the wife started me on this whole journey with a Mr. Beer kit, she also bought a 5g aluminum pot. So once I discovered how much I loved the hobby, I bought a full starter kit from Midwest with the fermenting buckets and all that so I could brew 5 gallon kits. The 5g pot was fine for this since I was only doing partial boils, but I wanted to do more, so I bought a 10g aluminum pot and an outdoor propane burner from Amazon. This allowed me to do full boils, partial mashes, and now all-grain brew in a bag. The 5g pot is still well-used in my brewing process as I put my squeezed grain bag into so I can get a few more cups of sweet sweet wort while I bring the wort to boil.
The next purchase I made was a small refrigerator and a 2 keg setup. It was expensive, but totally worth it. I despised bottling and I love kegging. Transfer the beer after its done fermenting into a keg, connect it to CO2 for a week, and enjoy your beer. I love coming home from a long day at work and pulling a nice draft from my keg fridge.
Next, I made a homemade wort chiller. I bought 25' of copper tubing from Home Depot and wrapped it around a keg and carefully bent the ends to the top. I clamped on some vinyl tubing and bought a garden hose/sink adapter, clamped it all together, and I had an inexpensive way to get 5.5 gallons of hot wort down to pitching temperature in about 15 minutes. I don't know if its improved the beer as much as it has just saved me a lot of time on my brew day and money from having to buy a bunch of ice.
I'd say my best purchase was the 10g pot and propane burner as it really opened up the ability for me to brew almost anything I wanted to for a 5 gallon batch. If you already have that, then temperature control is incredibly important. But, I have a basement like yours. Unless it gets to 100 degrees outside, my basement is always at 68 degrees. I keep a digital thermometer down there which has a high and low memory, with high at 68.6 and low at 68.2. I can live with that much variation, so I invested in a kegging setup and couldn't be happier.