When I bought my Husqvana I went to a dealer. I wanted a tractor that would last as long as possible. Now I realize that a $3,000.00 tractor is just that, and when buying tractors you get what you pay for. Not all of us can afford or justify 5, 6, 7,000 dollars or more on a tractor so we go for the best bang for our buck.
Anyway, I found that the big box stores only carry the lighter duty units. The dealer carries a full line. In my case I went for the top GT made by Husqvana. I like GT's because they tend to be heavier duty than a LT and hopefully last longer.
My dealer even filled out my warranty card for me. Keep in mind that even if you buy at a big box store they may not stock the parts you'll need. In fact, they may not sell your brand next year. Yes, you can always buy on line, but there are those times you need the part yesterday, a dealer is the best bet.
I have always done my own work and that is why I have joined this forum so that I can address the issues that I know nothing about. People here are very helpful.
As for what brand, go for the major brands like Rivets said. That way you can expect parts to be available etc.
Good luck