I've purchased many (a ridiculous amount of) treadmills over the years. You get what you pay for. Historically, I would buy an inexpensive treadmill at Costco or Walmart or Dick's Sporting Goods (around $1000), but those ALL inevitably died on me in 2 years or even sooner. Repair was usually possible but I couldn't ever get myself to spend $500 to repair a $1000 treadmill. So I'd just buy a new one.
I finally got sick of this cycle and invested in a Precor TRM 445. It's expensive but I love it and the higher quality is obvious in every aspect of the treadmill. The tread feels much gentler on my joints (like wow! What a difference), the tread is smooth, the machine is powerful. From all of my research I anticipate this machine to last a decade. The owner at the gym equipment store where I bought my treadmill has trained for marathons and other races using the store's Precor TRM 445 floor model.
Is $4000-$5000 a lot to spend? Yes! But my treadmill walking is my therapy in cold, gloomy winter months. I put a lot of mileage on my treadmills! It's my most valued piece of fitness equipment, second only to my weights. I consider it an "investment," so I saved for it.
Brands I think are great, based on my research, are Precor, Woodway, and the Landice L8 (the L7 doesn't seem as good). Look for at least 3.0CHP. A higher CHP is great, but often requires a special power outlet to operate.
Let us know what you end up purchasing.