Cris, It is possible to get reasonable moon shots at 250mm, but you have to crop a long way to get any sort of scale. You really want a minimum of 600mm, and ideally at least 1200mm with an SLR to get an image you don't need to crop. You can do widefield with a static tripod (See
this I wrote about it). You can also do startrails as indicated. Anything else and you are going to need to buy more kit, possibly lots of it.
For Deep sky imaging, you really need a heavy duty tracking mount. A 600mm ED refractor with an SLR is a good combination for the larger targets. To get some of the faint information, you really need a good guiding system, that's a second camera (which can do double duty as a high speed lunar/planetary camera) and a computer, or a specialised guiding camera with all the electronics attached.
For lunar/planetary, you really want a very long focal length, something above 3000mm, if not more. You will be operating at something like f/30 or more (they are diffraction limited optics, so even f/75, I've used it, isn't a problem, although getting enough light to the camera can be). You can get away with the same decent refractor (I am), but it's not ideal.
If you're wanting to do any form of imaging of our nearest star, you need specialist filters to cut 99.99999% of incident light (not expensive, I used some for my 55-250 with a pair of 58mm UV filters. Works a treat, but as the apparent size of the sun is the same as the moon, at 250mm it's pretty small in the frame) or you'll cook something, fast.
The single biggest expense for deep sky imaging is the mount. If it can't track accurately, and isn't stable enough, you won't get decent results. The load limit of the mount, is the upper limit and the thinking is that you want to be at about half the limit. If you put an expensive and stunningly good scope on top of a wobbly mount, you won't get better images, you'll probably get worse as the mount can't cope.
With a decent mount, you can use any lenses you might have, you'll be surprised just how big some targets are... my favourite image, I've produced, was shot with my nifty fifty, tracked with 33x5 minute exposures.
