$350 (more or less) is a good ballpark budget to get into refurbished DSLRs. I generally see more availability for refurbished Canons than Nikons around this price point.

https://shop.usa.canon.com/shop/en/catalog/cameras/refurbished-eos-interchangeable-lens-cameras#facet:&productBeginIndex:0&orderBy:11&pageView:grid&pageSize:&

long exposure [...] 5-15 minutes would be awesome

Most cameras offer 1-4000-8000th of a second (very fast) shutter speeds out to 30 second long exposures. Beyond that, you can easily set the shutter to "Bulb" and use an external intervalometer like this $20 one that I have (and love) for much longer exposures. You can also use it to shoot time lapse sequences,as a specific timer, or just to reduce camera movement for low light work by moving the trigger away from the camera.