I'm sure there will be differing opinions on this but I'm hoping that sharing my experience will give you something to think about before purchasing a trailer for your build base.
I bought the Tractor Supply 2000 lb trailer a few years ago as a base for my build. It was on sale and considerably cheaper than the NH bolt together kit; and it came assembled. I "used to be" all about shortcuts.
My TTT is in "camp-able" condition and I have had it out a few times without issue. Last weekend, during a trip down a filling-loosening, wickedly rutty road, I saw that the trailer was pivoting and twisting on the tongue fiercely. The tongue itself is connected to the trailer by only one bolt through a two sided bracket under the frame. Obviously there's going to be some pivot with this type of arrangement. Well, obvious to anyone a little more observant than I am, I guess.
Anyway, I mounted a storage box to the front of the camper with some stout wood screws during the build and then two brackets to connect it to the tongue. After a trip down that God-awful road, the pivot and twist was enough to pull the box free of the camper and do some wood damage. Thankfully the tongue brackets held or I would have lost the entire box and everything in it; not to mention collateral damage.
I bought some 2x3 tubing to match the tongue over the weekend and I'll be welding supports from the tongue to the undercarriage this weekend but, having to do this, it would just seem to make a lot more sense to buy the NH kit that already has the supported tongue/frame. That 20' length of 2x3 was $80, (they don't sell anything short of a full length), not to mention my time and labor. That would have just about made up the difference in cost between the Tractor Supply trailer and the NH kit. What's even scarier is that that pivot and twist could also be cutting into that one bolt that holds the tongue on and may, eventually, cause it to shear. If you're using one of these Tractor Supply trailers, you might want to check the condition of the bolt. I had a nightmare about Nosty's Nook shedding its' tongue and going south while my truck was headed north. I woke up in a cold sweat. Don't let this happen to you.

As I said, a lot of you may have had much better results, but for those still contemplating the purchase of a base trailer, think about this before you buy. I'm contemplating a standie for a "next" project and I've decided to build my own base trailer from scratch. Then I'll know what to expect right from the get-go. This is one experience I've definitely learned from. Who said you can't teach an old dog new tricks?
Cheers,
Brian