I've never owned a direct drive trainer, but yeah, they essentially replace the rear wheel, so I don't know why you wouldn't just use your existing TA.
I haven't shopped a trainer in earnest in several years, so I'm not aware of what the current market looks like. I do know that Kicker came out with a DD trainer a year or two ago that brought the entry level price of DD trainers down quite significantly; not sure how many other manufacturers have followed suit. Regardless, though, I'd probably check with DC Rainmaker - he's always updating trainer comparisons and reviews and should have some good info.