Looking to pick up a used treadmill, budget is about $300. Any ideas on what I should be looking for or staying away from. Buy once cry once, you get what you pay for and all that... I get. In all honesty, if the thing lasted me 6 months to a year and helped me reach my goal, that's really all I want. I don't need it to last forever.

I'm not a runner at all. I might be attending a LE academy and I'm trying to prepare for a 1.5 mile run 3x a week. Would use the street but trying to keep impact down as I'm 46 years old and have had back issues develop when I've run on the street in the past.

Just want to avoid injuries at all costs.

I don't have any specific advise for treadmills, but am curious as to where the running will take place at your LE academy.  If it is outdoors and on the street, why not start there anyway?  In my experience a treadmill won't me much less impact than street running anyway.  If you do use the treadmill for preparation, be sure to set the incline up a few degrees to help replicate the resistance that "normal" running has so you aren't setting yourself up for failure.