What are the dimensions of this?

It's about 12 inches deep 12 inches wide and 20 tall. Plus the side pockets which are another 2 or 3 inches on each side (big enough to hold a nalgene bottle in each side pocket). It's a 50 liter pack so its pretty big. It would be probably considered a "three day pack" meaning you could store enough enough stuff for three days of field combat/ camping etc. I love the pack. The build quality is good and has a plethora of pockets.. Plus its "molle" so you could always add more pockets on the outside...but the buckles could be stronger.

Can i add velcro to the front panel between the molle strip? or can i slip a coat/ jacket under the front panel?

No the front panel is not actually a pocket, its just for show. You cant slide anything underneath it. But as the other user said, you could tie some 550 cord around a loop somewhere and hang you jacket off of that.

Is there an internal pocket for water bag

Yes inside back pouch of main compartment

Does this have straps across the bottom to lash things to it?

No, there are no bottom straps. The top strap is the same as your shoulder harness tension straps to so strapping to the top doesn't work very well either.