What images can I expect to get from this setup?

[Mostafa]

I was wondering what images of dso I could get with this setup. How and how much would my images improve if I added in a decent file flattened and what would happen if I exceeded the maximum telescope magnification with Barlow lens.

[CraigT82]

Have a look on the Astrobin website, you can search the images using the terms "72ED" and/or "DSLR", that will give you an idea of what's capable with that scope and camera. Be sure to look at what mount was used though! 

[jambouk]

The mount is the limiting factor in that set up. I suspect you’ll be limited to 30-60 second subs depending on your polar alignment and acceptable of rejection rate of subs. 
 

I think a field flattener is good, but I would go for a better mount first.

[Stub Mandrel]

EQ3 needs careful setting up to give its best and a more rigid (or modified) tripod, but if you get up to 60 second unguided subs you can add guiding and easily achieve 5 minutes.

[ollypenrice]

Guiding is the life blood of astrophotography.

Olly

[alacant]

5 hours ago, Mostafa said:

what images of dso I could get with this setup.

Hi

Not sure where you are but from 38° N at the moment you could get m31, the Andromeda galaxy, and whilst you're waiting for it to get high enough, choose a star cluster in Cygnus. Add to m31 each night whilst choosing a different cluster beforehand. Even better with a field flattener.

HTH