Please don't assume you're alone in making what turns out to be a bad vehicle choice post-purchase. I'm dealing with that same issue body-wise on a current uber-reliable work van--calling it a POS (body wise) is an insult to P'sOS!

After initial visual inspection the first few turns of the engine can be quite revealing---hesitations to start just something to note. Engaging the transmission at an idle brakes being held shift between gears noting how quickly and solidly it feels.

Crawling underneath to note general condition including "cleanliness" for lack of a better term. Signs more important components like steering parts, brake calipers in general or other power train parts look to have been serviced at some point.

Tires especially if brand new and an off brand or cheap type could be covering up something---skepticism can be a good thing some days. Leaking grease under the hub caps, wheel studs very rusty are other signs to note.

Test driving after engine has warmed to operating temperature with a bit of aggressive driving can also reveal things that might need attention at some point.

Another POV to adapt is how you perceive or accept a used vehicle after its yours. Its a gamble, being prepared to get rid of it if it turns out repair costs will exceed any practical expense for the utility it would be.

As an example I buy vans I can pay cash for, one time and its all mine. Being self employed and 100% dependent on being mobile for income I've set aside enough money to completely replace any vehicle ASAP, no hesitation or lamenting what was. Additionally I have a second van that can immediately be pressed into service for some time and not miss an hour of work.

Working vehicles are simply tools, they sometimes break and have to be repaired or replaced. Having yourself situated to afford either option along with being at least 100% redundant gives such peace of mind its pretty amazing. I've had to work and plan carefully getting to my current status, at times that being challenging but once there keeping that status quo is far easier than you can believe---at that point its a matter of self-discipline. Easier said than done but doable anyway.

In short walk away from ANY deal that seems sketchy---your budget will thank you!

HTH