I had a Coleman popup (different than the new Coleman TTs) and I have a Keystone Outback now, which is our second Outback. Both Outbacks have been absolutely fantastic and I wouldn't hesitate to buy another.
The price of that Bullet should be a few thousand less than what you've been quoted. Keystone sells a ton of trailers, and like every manufacturer there are some that aren't as good, sometimes right off the same assembly line. It's been my experience to look over each individual new unit very thoroughly. For example, when we bought our current trailer, the one next to it (same year, make, etc.) didn't look as good as the one we actually bought (little things like the slide out seal wasn't as tight, etc.). Stupid, but true in a lot of instances.
For used units, you're at the mercy of your good common sense since you rarely know the history of what you're buying and are subject to a sales pitch, or how much the seller wants you to know. Now, that doesn't always mean you can't get a great deal, but you just have to be more of a detective there and devise your own checklist to ensure the big stuff looks as good as can be (slide outs work as designed, no delamination/warping of exterior, no water damage inside, etc.). There are many great checklists online for this, so arm yourself with a good one for peace of mind checking out used units. (And, this would also apply for new ones, too).
I actually like the new Coleman TTs a lot. Saw them at an RV show last year. Really impressed with what you get for that price-point. If I could only choose between the Coleman and used Bullet, I'd probably go with the new Coleman, because it's new and you have a warranty at least for some items for the first year.
Make sure you really do your research because there are a lot of great deals on new units out there if you're ultimately looking in that ~$20k range. The markups on new trailers are NOT what you actually end up paying for them, so keep that in mind when you look. I've often mentioned checking with Holman and Lakeshore RV on here when looking at TTs online because they tend to have the best prices around and they deliver. If nothing else, they can help you know a true retail price and help you in your local negotiations if you end up going new.
Hope this helps somewhat! Best of luck in your search!